1. The evolution of transport systems in plants
Introduction
Xylem
Phloem
Summation; integration

2. evolution or revolution ?
In the beginning………….
carbon
oxygen
ozone
nitrogen
light
hydrogen
migration
air
hydrocarbons ?
stress ? ?
water ?
land

3. The symplast and apoplast
Symplast - Defined as that region of the living plant, that is bounded (enclosed by) a plasmamembrane. The cells forming this group are called a DOMAIN, and DOMAINS are connected via PLASMODESMATA.
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Apoplast. By definition, all regions of living plant cells NOT bounded by a plasmamembrane. This must include the CELL WALL as well as intercellular spaces. These two constitute FREE SPACE in the plant. Apoplast thus involved in the (free) movement of substances and the principal conduit in this case, must be the xylem or xylem-associated cells that LACK a plasmamembrane.

4. xylem and phloem - the parallels
Phloem and xylem have distinct physiological functions, and their distinctly-different functions give them a unique parallelism.
The xylem conducts water, usually acropetally, from root to shoot and the leaf, (in a sense, from source to sink) whilst the phloem in the crudest sense, transports carbohydrate and other substances from a site of manufacture (source) to a site of utilization (sink).
In both cases, tubes are involved – sieve tubes in the phloem, and xylem vessels and narrower trachieds in the xylem. Both systems are associated with parenchymatous elements, and both are relatively delicate structures.
It is now more than 160 years since, for example, the publication of Hartig’s descriptions of the bark of trees. There is much new information concerning structure and function, but also, we have become more aware of the fossil record, allowing us to trace the evolution of these remarkable conduits through some 500 million years of evolution.
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5. phloem - early work
1. In 1837, Theodor Hartig published his work on the organization of the trunks of forest trees, and commented on the then “completely uninvestigated elementary organs” of what was called the ‘sap skin’ (Safthaut) of trees.
2. Significantly, Hartig described three types of cell – Siebfasern, Siebröhren and keulenförmige Saftröhen – in English, the sieve elements and laticifers common in Euphorbia for example.
3. The Siebröhren and Siebfasern correspond entirely to what we describe today as sieve tubes and in the latter case, to sieve cells, associated cambial cells, phloem parenchyma as well as sclerenchymatous elements.
4. In 1858, Nägeli coined the term ‘phloem’ top describe the food conducting systems in higher plants.
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6. Moving on. Developing the xylem transport system
Initially, transport of water in plants that had just emerged onto land, would have been a physiological necessity –
Why?
Even though these plants would have been tiny, it would be advantageous to be able to move water fairly quickly, to ensure that the uptake of nutrient (particularly organic and inorganic ions) would be facilitated from the soil environment. Loss of water through non-specialised pores and later through regulated stomates, would have facilitated cooling as well as the regulation of the rate of transpirational water loss.

7. Ancient origins
Indeterminate hydroid or stromatoporoids in Starostinella nordica
Upper Permian ca 280 MY
Since then many examples in which hydroids appear.
Starostinella nordica gen. et sp. n. is described from the uppermost Permian (Kapp Starostin Formation) of the Kapp Starostin (Isfjorden) in West Spitsbergen. The new genus is attributed to Trachypsammiidae Gerth - a family incertae sedis among Cnidaria.
Members of the Trachypsammiidae have been previously associated with different higher rank taxa within the Cnidaria, or their skeletons were interpreted as a result of symbiosis of a cladochonoidal organism (Tabulata) with an indeterminate hydroid or stromatoporoid. S. nordica gen. et sp. n. seems to support the latter assumption.
Hydrocoralla of S. nordica have a simpler structure than those of other Trachypsammiidae and are branching like those of Cladochonus. Their thick-walled, horn-shaped hydrocorallites are surrounded with a very thick cortical zone of sclerenchyma organized into trabecular microstructure. The proper corallite wall is fibro-radial in structure, sharply distinct from the outer cortical zone
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NOWINSKI-ALEKSANDER . A new trachypsammiid Cnidarian from the late Permian of Spitsbergen. Polish Polar Research. 18[3-4],

8. Stelar arrangements and tissue organisation
All comparisons serve to underscore the fundamental relationship in early vascular plants between the evolution of increased complexity in stelar architectures and the evolution of complex lateral branches and leaves.
water transporting function is the crucial factor in the evolution of the various steles.
Vascular plants seem to be the ideal study group for integrating developmental process directly into analysis of homology and estimation of phylogenetic relationship.
STEIN, W. Modeling the evolution of stelar architecture in vascular plants. INTERNATIONAL JOURNAL OF PLANT SCIENCES 154,
The author compares modelling auxin level results, to members of the Iridopteridales, Cladoxylopsida, Calamopityaceae, and Medullosales. He claims that there is striking correspondence which allows for reinterpretation of the evolution of vascular plant stelar architecture not just in terms of historical patterns of important mature structures but also as a system of evolving developmental dynamics underlying these structures. All comparisons serve to underscore the fundamental relationship in early vascular plants between the evolution of increased complexity in stelar architectures and the evolution of complex lateral branches and leaves. Among evolutionary events occurring in the fossil record, the model of hormone determination offers important insights into
origin of ribbed protostelic systems from primitive columnar architectures as a consequence of increased compactness of the shoot apex and orthostichous phyllotaxis;
conspicuous differences in the three-dimensional configuration of protoxylem strands supporting a developmental distinction between Devonian "radiate protoxylem" and "permanent protoxylem" groups;
quantifiable differences in protoxylem/metaxylem tissue fabric probably related to differences in hormonal activity of lateral appendage primordia during early development;
origin of pith at the centre of the stele related to changes in the geometry of the shoot apex under several possible models of hormone determination; and
dissection of the stele into discrete primary vascular bundles possibly related to changing receptivity to the hormone signal and the geometric consequences of flow rates in three dimensions over developmentally significant intervals of time.
Stein argues that it is possible to interpret evolutionary change within and between major groups of vascular plants. Because of the unique nature of their development and excellent fossil record, he suggests that vascular plants may be the ideal study group for integrating developmental process directly into analysis of homology and estimation of phylogenetic relationship.

9. summary: main evolutionary trends TRACHEID DIMORPHISM 1
pit membrane diameter decreases
reduction in size of borders on pits
fewer pits
tracheid
fiber tracheid
libriform fiber

10. summary: main evolutionary trends TRACHEID DIMORPHISM 2
fiber tracheid
libriform fiber
SPECIAL EVOLUTIONARY TRENDS
Trends in the evolution of imperforate tracheary elements are illustrated in the above figure. The trend that leads from tracheids to libriform fibers represents also a division of labour between conductively more efficient elements, and mechanically more sound (better) elements is basic to the scheme illustrated above.
Thick-walled narrow latewood tracheids are mechanically superior to early wood tracheids (Boatwright and Garrett 1983) but conductively poorer. Mainte­nance of higher conductive capacity can be achieved, therefore, by alternate pro­duction of early wood and latewood in seasonal environments. Uniformly moderately thick-walled tracheids in relatively nonseasonal environments (e.g., Agathis, Araucaria) are adaptive also, however. Longer tracheids are stronger than shorter ones (Wellwood 1962) as well as superior in conductive capacity. In smaller conifers, mechanical strength is probably not lowered by decreasing cell size, al­though conductive capacity probably is lowered. In other words, the feature that explains the selective value of longer tracheids is the increase in conductive capacity of longer end-wall overlap areas. This explains why there is a sharp drop in length of all tracheary elements (both imperforate tracheary elements and vessel elements) with the origin of vessels (Carlquist 1975a, p.141).
The origin of the perforation plate meant that there is no longer any selective value for long tracheids (to preserve greater conductive area per cell)
Vessel-Bearing Dicotyledons
The I. W. Bailey and Tupper (1918) data show that vessel-bearing dicotyledons have a wide range of vessel-element lengths, and that shorter vessel elements oc­cur in more specialized groups (groups with more numerous specialized floral fea­tures). Therefore, there has been a continued drop in length of fusiform cambial initials during phylesis of woody dicotyledons. However, that tendency is not readily explainable on the same bases as those that have produced the drop in tra­cheid length over time. Yet data do show that vessel elements are shorter in vessel bearing dicotyledons of drier habitats compared with their relatives in wetter habitats. This is most conveniently shown in a single large family, such as Asteraceae (Carlquist 1966 a) or a genus such as Erythroxylum (Rury 1985). Shortening of vessel elements in drier habitats can also be shown on a floristic basis (Novruzova 1968, Carlquist 1977 d, Baas et al. 1983, C'arlquist and Hoekman 1985 b, Barajas­Morales 1985).What has caused the reduction in length of fusiform cambial initials once vessels originated?
If related to xeromorphy, several possibilities are available. Shorter vessel elements might be stronger because of the constriction formed by each end wall (even if it contains a simple perforation plate). Presence of these vessel constrictions could resist deformations related to tensions in water columns (Carlquist 1975a). However, another more appealing possibility is that short vessel elements are valuable because they localize air embolisms to a greater ex­tent than long ones because of the constrictions formed by perforation plates, even simple perforation plates (Carlquist 1982 c). Slatyer (1967) stated that air bubbles should be so confined by vessel elements, even if perforation plates are simple. Root pressure may expel air bubbles from vessels in lianas and some other woody plants (O'Leary 1965, Putz 1983, Ewers 1985), but in many woody plants that does not happen. Evidence for localization of air bubbles in individual vessel elements has been provided by Sperry (1985), tending to confirm Slatyer's statement.
Woody plants that has simple perforation plates show that individual bubbles tend to be confined to individual vessel elements. Where a long bubble extending beyond a single vessel element occurred, I observed that the bubble shrank to the nearest perforation plate, and the tip of the bubble appeared confined to that perforation plate, even though no bars traversed the perforation plate.
When water stress occurs, air embolisms occur frequently in vessels and cause problems for the conductive system. This has been demonstrated effectively and repeatedly
(Milburn1973 a, b, M. H. Zimmermann and Milburn 1982, Tyree and Dixon 1986).
tracheid dimorphism:
vasicentric tracheids +
septate fiber tracheids
fiber tracheid dimorphism:
fiber tracheids + vasicentric tracheids
fiber dimorphism: parenchyma-like fibers (or parenchyma) + libriform fibers

11. Narrow is best ? Why? Mostly gymnosperm, some vesselless angiosperms
Tracheids in their various forms are thus the principal water conducting cells in gymnosperms, as well as in vesselless dicotyledons.
They present many advantages in terms of transport.
Think for example, of the boreal forests – what is the species composition?
Mostly gymnosperm, some vesselless angiosperms
click
Why?

12. Vessel bearing dicotyledon
Vessel elements and tracheids much shorter than those in vessel-less wood
Transition from vessel-less to vessel-bearing wood. Qualitatively, a marked drop in the length of the tracheid to primitive vessel occurs.
Vessel end retains remnant primary wall strands

13. Scalariform perforation plates
Scalariform perforation plates are indicative of the evolutionary level of the species, just as shortening of the vessel elements is a (reliable) factor indicative of evolutionary advancement, so to is the shape, structure and hence, morphology of the scalariform perforation plate associated with the end wall perforation of a vessel.
Bordered bars in primitive species vary but all are subdivided as in this example.

14. Overview of variation and evolutionary lines of scalariform perforation plates
evolutionary paths towards simplified perforation plates. Does this suggest environmental and cliamatalogical influences?

15. Variations and evolutionary lines of scalariform perforation plates 1.
scalariform = ladder-like structure
Perforation plates become simpler, but still flake- or strand-like. Advantages? Disadvantages? Climatology?

16. Variations and evolutionary lines of scalariform perforation plates 2.
coalescence of pores into larger structures.
Advantages? Disadvantages? Climatology?

17. Lateral wall pitting, in tracheids and vessels
opposite
alternate
scalariform
transitional
Scalariform pitting: Occurs where the pits are laterally elongate, and occur across a ‘face’ of the vessel, more especially in angular vessels. Can occur on vessel-ray interfaces as well, where contact with the ray will form and define the ‘face’. Scalariform pitting is not common in dicotyledons.
Transitional pitting: Occurs in vessels where there is a scalariform-like pattern, in which some of the pits extend across the defined face of the vessel, whereas at other points, two or three smaller pits are present, instead of one laterally-extended one. What is envisaged is that some of the laterally larger scalariform plates do not break up completely (bars of primary wall remain attached, and encapsulated by membrane material for example, and as such, secondary wall material is rapidly deposited in these small areas.
Transitional pitting is not common in dicotyledons either.
Opposite pitting: Here the smaller, generally rounder pits are formed as lateral series on vessel walls. Pits occur in clearly-defined rows (or lines) traversing the vessel. Again, pitting is present in few dicotyledons, commonly seen in some members of the Magnoliaceae.
Alternate pitting: By far the most commonly seen pitting in dicotyledons. Overall pattern is helical.
Paedomorphosis: Phenomena, which collectively indicate that age on length criteria vary as a result of decreased length of the fusiform cells. This decline in fusiform cell length is reversed in may species as secondary xylem vessels become longer. Tend to occur in species with specialised characteristics such as Asteraceae or Campanulaceae. They therefore tend to have short vessel elements when compared to primitive woody families. However, Carlquist states that paedomorphic species tend to have ‘longer’ vessel elements, but that the gradual shortening of the wood may not have a direct functional explanation, but could be an expression of a relaxation of the expression of mechanical strength. Paedomorphosis could in many species in which this phenomenon occurs, represent minimal elongation of the fusiform initials and result therefore, in minimal selection for mechanical strength.
Neotony juvenilism (features more often found in primary xylem than in secondary xylem).

18. The bordered pit – micro-engineered flow control
Once the pit membrane pores are blocked, flow is impeded
minimum air bubble size is larger than the size of the pit membrane pores
The combined flow capacity of the pit membrane pores, equals that of the pit aperture; in other words, efficient traffic through the bordered pit requires that the sum of the area of the perforations in the pit equal the area of the border
weakening of the wall is lessened by overarching of the pit border.

19. Phylogenetic change: foreshortening vessels, dicotyledonous wood.
Vessel members usually have perforate end walls, but in steeply-inclined walls, these perforations are really on the side of the vessel element.
The perforation plates are varied and are described as scalariform (1,2 and part 3); mixed simple-scalariform (3); reticulate, foraminate and simple.
There are several principles defined by Frost and explained in Fahn, that govern the way in which we can define, describe and perhaps understand the phylogenetic processes concerning the origin of dicotyledonous vessels.
The most widely accepted is the association method: This method states that it is possible to determine which of two structures is the more primitive, and if it is assumed that the two structures have a direct genetic relationship, it will be possible to assume that the more primitive character or condition in the more advanced structure, will be similar in general condition to the same character in the more primitive structure. If there is no such similarity, then it would be incorrect to assume a relationship as the elements in question are in fact, too far removed from one another to allow this. With reference to tracheary elements, it is assumed that there is a phylogenetic relationship between tracheids and vessels.
Short, wide vessels are very efficient transporters, but extremely prone to embolism.

20. Phylogenetic change: monocotyledonous wood
Important to remember that vessels first appeared in the root, only later in stems and leaves.
Specialization of these, followed the same pattern
Bailey, 1944:

21. summary: main evolutionary trends TRACHEID DIMORPHISM 3 loosing vessels
Alternative explanations for shortening of vessel elements can be imagined, although there are at present few data to support them. For example, one can sup­pose that narrow vessels are of selective value because they embolize less readily-a fact that has been demonstrated (Ellmore and Ewers 1985). Production of narrow vessel elements could be hypothesized to lead to production of shorter vessel elements if there is a correlation between diameter and length morphogenetically. But as one can observe in a growth ring of any ring-porous species, narrow vessel elements can be achieved without shortening of vessel elements; in fact, late wood vessel elements are slightly longer than early wood vessels (Swamy et al. 1960).Shorter fusiform cambial initials could be hypothesized to be advantageous: divisions in a short fusiform cell (e.g., the vertical radial divisions in fusiform ini­tials of storied cambia) could take place more readily. However, the existence of long fusiform cambial initials in conifers counters that idea quite easily. Storied cambia have notably short fusiform cambial initials; there is not a strong correlation between storied cambia and dry habitats, whereas there is a strong corre­lation between short vessel elements and dry habitats. One could hypothesize that vessel element dimensions are related to the size of plants in which they Occur. However, in the southern Californian flora, shrubs have shorter vessel elements than trees, but herbs have longer vessel elements than shrubs (Carlquist and Hoekman 1985 b). One could hypothesize that shortening of fusiform cambial initials in dicotyledons is related to mechanical considerations, and that length of imperforate tra­cheary elements, rather than length of vessel elements, is basic to the trend. Be­cause of intrusiveness of cambial derivatives destined to become imperforate tra­cheary elements, very long cells can be achieved from products of relatively short fusiform cambial initials. The mean length of imperforate tracheary elements in dicotyledons (1317 um) is about twice the mean length of vessel elements in dicotyledons (649 um) according to the data of Metcalfe and Chalk (1950, pp. 1360­1361), so the average dicotyledon is not achieving long mechanical cells by having long fusiform cambial initials, but by intrusive growth of derivatives of the fusi­form cambial initials. Length of mechanical cells in wood is probably not as significant a factor as wall thickness, wall chemistry, wall ultrastructure, or amount of mechanical tissue in providing mechanical strength (Boatwright and Garrett1983).
Vessel dimorphism leads to the formation of libriform vessel elements, and eventually, vasicentric tracheids. Tracheid dimorphism and fiber tracheid dimorphism
Defn: vasicentric =

22. Trends in the evolution of vessel elements
vessel element with scalariform perforation plate; plus tracheid
SAFE SYSTEM
Evolutionary adaptation
The scheme is based upon an assumption that in primitive wood, vessel elements with scalariform perforation plates, will usually tend to be associated with tracheids. In contrast, more specialized wood in which there are vessels with simple perforation plates, will be associated with libriform fibers.
Carlquist acknowledges that deviations from this (simplistic?) notion will occur, the three stages represented above (taken from his book, see Ch 11. Fig. 11.1) do exist and do represent a progressive increase in the division of labour between conductive efficiency and mechanically optimal cells.
vessel element with nearly simple perforation plate and a fiber-tracheid
vessel element with a simple perforation plate; accompanied by libriform fiber
IMPROVED CONDUCTIVE EFFICIENCY
IMPROVED MECHANICAL STRENGTH
DECREASED SAFETY
libriform fibre which has pits with no border and a slit-like aperture on the outer face.

23. towards conductive efficiency and safety #1
vessel element with scalariform perforation plate; plus tracheid, safe wood
Vessel element with simple perforation plate provides conductive efficiency. True tracheids provide & retains conductive safety
Scalariform = ladder-like

25. towards conductive efficiency and safety #2
vessel element with nearly simple perforation plate and a fiber-tracheid
Vessel element with simple perforation plate provides conductive efficiency. vasicentric tracheids and fiber tracheid provide & retains conductive safety

26. towards conductive efficiency and safety #3
vessel element with a simple perforation plate, accompanied by libriform fiber
Vessel element with simple perforation plate provides conductive efficiency. vasicentric tracheids and libriform fiber provide & retains conductive safety
libriform: In secondary xylem, with few, simple pits. Slit-like aperture in the outer face

27. pumpkin
sunflower

28. Tracheary element evolution – (& the vesselless dicotyledon)
Narrower tracheids have circular pits on end and lateral walls
Wider tracheids may bear scalariform pits on overlap areas (end walls), as well as on their lateral walls.
Evidence exists that amongst woody dicotyledons that vessels first originated in the secondary xylem, and then, with time, progresses backwards to the primary xylem.
Imagine the changes necessary to convert from vessel-less to vessel containing wood: Tracheary elements must become shorter – the evolving vessels are derived from wider tracheids which as in the illustration above, bear scalariform perforation plates and pitting on their end walls. Another aspect commonly referred to is that there appears to be a tendency for acutely-angled end wall contact areas between emergent tracheid-vessels in more primitive species and families than in the more advanced ones.
Wall structure
Most attempts to understand the early evolution of tracheids have cantered on fossil Silurian and Devonian vascular plants, and these efforts have led to a wealth of new information on early water-conducting cells. All of these early tracheids appear to possess secondary cell wall thickenings composed of two distinct layers a layer adjacent to the primary cell wall that is prone to degradation (presumably during the process of fossilization) and a degradation-resistant (possibly lignified) layer next to the cell lumen. Developmental studies of secondary wall formation in tracheary elements of extant vascular plants have been confined to highly derived seed plants, and it is evident that the basic structure of these secondary cell wall thickenings does not correspond well to those of tracheids of the Late Silurian and Early Devonian.
Significantly, secondary cell wall thickenings of tracheary elements of seed plants are not known to display the coupled degradation-prone and degradation-resistant layers characteristic of tracheids in early tracheophytes. We report a previously unknown pattern of cell wall formation in the tracheids of a living plant. In Huperzia, one of the most primitive extant vascular plants, secondary cell wall deposition in tracheids includes a first-formed layer of wall material that is degradation-prone ("template layer") and a later-formed degradation-resistant layer ("resistant layer"). These layers match precisely the pattern of wall thickenings in the tracheids of early fossil vascular plants and provide an evolutionary link between tracheids of living vascular plants and those of their earliest fossil ancestors. Moreover, developmental data provide the essential information for an explicit model of the early evolution of tracheid secondary wall thickenings. Finally, congruence of tracheid structure in extant Huperzia and Late Silurian and Early Devonian vascular plants supports the hypothesis of a single origin of tracheids in land plants.
Cook ME and WE Friedman, 1999? Tracheid structure in a primitive extant plant provides an evolutionary link to earliest fossil tracheids
Tracheid end wall. These are really micropores in the pit membrane (scalariform perforations develop from this).
Are there advantages? disadvantages? Much argument still.
Example? Winteraceae resist freeze-thaw conditions, 0 – 6% loss of hydraulic conductance only, vs >20% for vessel-containing dicots. Clearly an advantage. Reasons for the possible loss of vessels and the ecological events underlying this process remains mysterious!

29. 50 µm
50 µm
pine, TS
pine, LS

30. Whose got xylem?
Division Psilophyta: Psilopsids, characterised by presence of microphylls. No differentiation of shoot and root.
Division Lycophyta: Have microphylls.
Division Sphenophyta: The horsetails Single genus (Equisetum) – jointed stems, conspicuous nodes. Leaves scale-like.
Division Pterophyta: Ferns. All possess a megaphyll.
Division Coniferophyta: Conifers. Gymnosperms with active cambial growth (secondary tissue) simple leaves.
Division Cycadophyta: Cycads. Gymnosperms with sluggish cambial activity.
Division Ginkophyta: Considerable cambial growth, fan-shaped leaves. Open dichotomous venation.
Division Gnetophyta: Gymnosperms with many angiosperm-like features. Only Gymnosperms in which vessels occur.
Division Anthophyta: Flowering plants. Megaphylls, secondary growth. Contain dicotyledons (cambium) and monocotyledons (no cambium).

31. 50 µm
200 µm
maize
Helianthus
pine

32. Summary
First land plants had hydroids – simple in structure – may have been lignified
Later evolution of more complex wall structure
Evolution of tracheoids
Tracheids become the principal water conducting elements
Vessels – essentially complex, with compound perforations (scalariform common)
Mesomorphs/subtropical to tropical wood – evolution of simple perforation plates, some crassulae
Crassulae: Transversely oriented thickenings in tracheid walls of gymnosperms accompanying the pit pairs. Also called Bars of Sanio).
Shortening and widening of vessels in warmer climates.
Essentially, the Winteraceae are traditionally regarded as the least-specialized descendents of the first flowering plants, based largely on their lack of xylem vessels. Since vessels have been viewed as a key innovation for angiosperm diversification, Winteraceae have been portrayed as declining relicts, limited to wet forest habitats where their tracheid-based wood does not impose significant hydraulic constraints.
In contrast, phylogenetic analyses place Winteraceae among angiosperm clades with vessels, indicating that their vesselless wood is derived rather than primitive, whereas extension of the Winteraceae fossil record into the Early Cretaceous suggests a more complex ecological history than has been deduced from their current distribution. But the reasons for the possible loss of vessels and the ecological events underlying this process remains mysterious.

33. Xylem transport Xylem Transport Introduction
It is not possible to present a simple, comprehensive model to demonstrate the wide range of arrangements of vascular systems that occur in either dicotyledons, monocotyledons or gymnosperms in a few lectures. In dicotyledons that are composed of wholly primary tissues, structures tend to be a little more stereotyped than monocotyledons, but even then there is a very wide range of arrangements.
The essential elements of both systems are the xylem, concerned with transport of water and dissolved salts, and the phloem, which translocates synthesised but soluble materials around the plant, to places of active growth or regions of storage.The xylem is the principal conduit for long-distance water transport in higher plants.
In most dicotyledons the leaf lamina has a midrib to which are connected the lateral veins. The latter form a network composed of major and minor systems. The midrib is directly connected to the petiole trace. This enters the stem and joins into the main stem system through a leaf trace gap as described above. In the primary stem, all vascular bundles are separate from one another (indeed, they remain separate in many climbers, e.g. Cucurbita, Ecballium), but in most dicotyledons, the bundles become joined into a cylinder by growth of secondary xylem and phloem from the fascicular and interfascicular cambia.
A complex rearrangement of tissues takes place in the primary plant where the systems of the stem and root meet. In the stem vascular bundles, the phloem is normally to the outer side of the xylem in the majority of plants. In the root, the xylem is central, and may have several lobes or poles, with the phloem situated between these. The transition region between stem and root is called the hypocotyl. After secondary growth has taken place, this complex zone becomes surrounded by secondary xylem and phloem, and the shoot and root anatomy become more similar.
Transfer cells are specialised parenchymatous cells found in various parts of the plant, but in particular, in regions where there is a physiological demand for transport, but where more normal phloem or xylem cells are not in evidence. A good example is the junction between cotyledons and the shoot axis in seedlings. Transfer cells may also be present near the extremities of veins, or near to adventitious buds, for example. Thin sections of the walls of transfer cells show them to have numerous small projections directed towards the cell lumen. These greatly increase the plasmalemma-cell wall interface a site of metabolic activity concerned with the rapid, energy-mediated movement of materials between adjacent cells. The projections are so fine that conventional sections with a rotary microtome are too thick for them to be seen.
Monocotyledons are quite different from dicotyledons in their vasculature. Leaf and stem, are commonly much less readily separable as distinct organs, thus jointly constitute the shoot. There is no secondary growth by a true cambium, so a cylinder of vascular tissue does not form. When secondary growth occurs, as in Agave and Cordyline, it is by means of specialized tissue, situated near to the stem surface, which forms complete, individual vascular strands and additional ground tissue.
The tissue is complex, in that it contains several distinct cell types, which include:-
Tracheary elements fibres and parenchymatous elements.
Xylem Transport
The Xylem is referred to as being ‘dead’ at maturity – this is not so, as many of the cells of the xylem are either parenchymatous (and therefore contain a full compliment of organelles) or if lignified, are living as well. These living cells the rays referred to under point 3 below, are vital communication pathways in higher plants.
Tracheary elements are defined at those cells that are involved primarily in water transport, and two types of cell are recognized:
Vessel elements. Vessels are recognized where a definitive end wall structure exists. That is, the end wall of the cells become modified, and reduced to just a rim, or the end wall is hydrolyzed during maturation, to leave a multiperforate structure, in its place. These elements are joined end to end, to form a tubular structure. Principal transport root through vessels is axial with a component of radial transfer possible as well. Side walls are usually perforate, and contain many pits in their walls. These pits coincide with other pit structures of associated and congruent cells, to form pit pairs through which water may travel as well. Consult the references for illustrations of pit pairs and look at slide # 7.
Tracheids. Tracheids are more primitive than vessels, are narrower in diameter, and have tapering ends. Their end walls overlap and thus tracheids have a secondary function of mechanical support (because of the overlap). The narrow diameter of tracheids means that there is greater resistance to water flow, but the narrow diameter of the capillaries that they form, means that the water column will not cavitate as easily as it will in say, a 100 µm diameter xylem vessel.
Parenchyma. Parenchyma cells may be come lignified (develop a secondary wall) as the plant matures. The parenchyma cells associated with vessels or tracheids, are implicated in transfer of solute from the axial transport system, to the radial system, from whence materials are shuttled (in water) to and from the phloem tissue. Recent experiments (illustrated in this presentation) have clearly demonstrated that transfer of passively-transported (carried in the water column) substances from the xylem vessels occurs via specialized hydrolyzed wall areas (see slides 10-11).
Copyright Warning:
Some of this material is subject to copyright and may form part of the text, ‘Plant Anatomy – an applied approach’, by DF Cutler, CEJ Botha and WD Stevenson. Any infringement (unlawful copying or re-use of any of this material without the author’s express permission) will be prosecuted through the contracted publishers, Blackwell's Scientific (UK).
You are advised to consult the following references:
Esau: Anatomy of Seed Plants
Biology of plants Raven Evert and Eichhorn
And the web-based The Virtual Plant

34. The phloem Definitions
Sieve element: The cell of the phloem that is specialized and involved in the long-distance transport of food substances; sieve tubes are further classified into sieve cells and sieve tube elements (or members).
Sieve cell: A long slender element, with relatively unspecialized sieve areas, with tapering end walls that lack sieve plates; found in the phloem of gymnosperms.
Sieve tube element (member): One of the component cells of the sieve tube; found primarily in the flowering plants and typically associated with companion cells.
Sieve plate: That part of the wall of sieve tube elements (members) bearing one or more highly differentiated sieve areas.

35. Sieve element evolution, brown algae
The micrograph at left, shows a longitudinal section of the phloem of Desmarestia ligulata. Sieve elements are trumpet-shaped where end walls join.
This plant belongs to one of the most dominant groups of seaweeds occurring in the Antarctic ocean.
Nereocystis lütkeana
This LS shows adjacent sieve elements of the pneumatocyst, in which files of sieve elements are separated by large spaces

36. organized transport in seaweeds sieve cells or sieve tubes
1. The perforated end wall is one of the most characteristic features of the phloem in the marine algae. The size of the perforations are species dependant.
Pore sizes range from as small as plasmodesma (100 nm or so) or as wide as the sieve pores in vascular plants.
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2. Phloem systems tend to be well-developed for long-distance transport of organic and inorganic nutrient.
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3. In Laminariales, cross-connections exist, interconnecting longitudinal strands either via cross-connecting sieve elements, or via lateral sieve areas. In Macrocystis these lateral sieve area pores are smaller in diameter than those occurring in the cross walls, thus one could define the sieve elements as true sieve tubes and the perforated end walls are thus true sieve plates.
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37. the phloem lower orders, Gnetophytes
Within the cycads, the phloem consists of sieve elements and narrow parenchyma cells.
The Cycadophytinia represent a group of plants, extant since the Jurassic (208my), with strong links to the angiospermae.
The phloem of the Gnetophytes is typically Gymnospermous, in that the sieve areas are similar to those found in other types of gymnosperms, and are not comparable to those found in the more advanced angiosperms. The sieve plate pores are much smaller than those in the typical angiosperms, never reaching much more than 0.1 m in diameter.
In the Gnetophytes, there are two distinct types of parenchymatous cells – the first of these is typically densely staining (referred to as ‘albuminous cells’) or more correctly Strasburger cells, which occur in regular radial files of cells, alternating with the sieve elements. Strasburger cells are interconnected by plasmodesmata with regular phloem parenchyma cells. There connections to sieve elements are intermediate between a sieve area and a plasmodesmata.
Note: The term ‘sieve element’ is used here, to describe more primitive sieve cells and more advanced sieve tube members.
Longitudinal sections through part of the phloem in Cycas revoluta, showing differentiated (SE) and differentiating sieve elements (DSE)

38. primitive dicotyledonous plants
The photomicrographs to the left, show details of the phloem tissue in the phloem of Magnolia kobus in which the cell wall of the sieve elements, can be see to be thickened, and of wavy outline. This wall is termed the nacreous wall, which is less cellulosic and pectin-poor compared with the outer wall layer. These inner walls have been shown to be polylamellate in many species.

39. higher plant sieve plate pores
As seen in this example from Beta vulgaris, the sieve plate pores in mature sieve elements (bottom micrograph) are large, and if prepared properly for electron microscopy, are devoid of callose. They are approximately 0.5 µm in diameter.
0.5 µm

40. unusual wall thickenings
This electron micrograph shows sieve elements in the monocotyledon, Heterozostera tasmanica Note the thickened sieve tube walls, termed nacreous thickening

41. fossil records, carboniferous (362my) gymnosperms
Left: Cordaixylon, ts outer phloem. Parenchyma cells appear smaller than the sieve cells (S).
Right: Callistophyton, showing alternation of sieve cells with parenchyma cells. R = Ray tissue
EL Taylor Phloem evolution: Ch 14, Behnke and Sjoland Sieve Elements

42. Evolutionary process, fossil records -- Lycopods, Sphenophyta and ferns
Taxon
Details of Phloem
Lycopods
Sieve elements, parenchyma; ca 15.7 µm xs; end wall horiz/obl;
Sphenophyta
Spenophyllum
Sieve elements; µm xs; end wall horizontal
Primary and secondary sieve elements + parenchyma + interfasicular parenchyma.
Ferns
Ankyopteris; Botryopteris; Elapteris; Psaronius; Sauropteris
Adaxial & abaxial sieve elements; small & large sieve elements, (small = µm; large = µm).
Sieve plates horiz – oblique; have sieve areas and sieve pores

43. Evolutionary process, fossil records - Progymnosperms
Taxon
Details of Phloem
Progymnosperms
‘aneurophyalean’
Sieve cells, µm xs; very oblique end walls; sieve pores v. small, µm
Callixylon (Devonian)
Not understood, poor preservation
Calamopitys
Sieve cells (primary and secondary);50-60 µm xs; oblique end walls
Callistophyton
Sieve cells; secretory canals; µm xs; very oblique end walls; v. long sieve cells (6mm).
Cordaites
Cordaixylon
Sieve cells 25 µm xs; cf 1.5 – 1.7 mm long; very tapered ends.
Coniferales
Cupressinocortex
Sieve cells; µm xs
Taxodioxylon
Sieve cells; 20 µm xs 50 µm long

44. Fossil records - cryptogram & gymnosperm phloem similarities
Tissue origin:
conducting cell:
tissue composition:
Vascular Cryptograms
sieve elements (primary)
sieve elements and parenchyma cells
Gymnosperms
sieve cells (primary and secondary)
sieve cells, parenchyma elements, fibers, sclereids, secretory cells
Sieve elements:
shape:
diameter:
length:
end wall:
sieve areas:
sieve pores:
callose:
rectangular to elongate
10-40 µm (up to 120 µm a)
<600 µm (>2.75 mm)
horizontal to very oblique
elliptical-rectangular & small
approx 1 µm dia.
present in some
elongate µm
1 – 9 mm
elliptical, larger app. 10x30µm

45. Phloem phylogeny #1
Amongst fossil and extant species, the basic structure of the sieve element is fairly uniform in the cryptograms,.
Primary phloem is very poorly known from fossil Gymnosperms and the data presented in the previous two tables is based purely on the secondary phloem.
Whilst phloem has been noted in all groups of fossil Gymnosperms, it appears to have been researched in very few groups, including the cordaites, conifers, and Palaeozoic seed ferns.
In all instances phloem alternates with bands of fibers which may have interspersed axial parenchyma.
All in all the gymnosperms, the sieve cell is remarkably constant in its structure, with the first Middle Devonian progymnosperm record, being very similar to those in Carboniferous seed plants, and are comparable to extant sieve cells being 2.5 -> 9mm in length, with gradually tapering end walls.

46. Phloem phylogeny #2
Regardless of the group within the Gymnosperms, there is an intimate and regular relationship between axial parenchyma and conducting elements.
Fibers tend to be present and are variable in distribution, from the innermost (presumed functional) to the outermost regions closely spatially associated with the bark.
Sieve areas are discrete, uniform in size and shape, with distinct pores that can be counted (unlike cryptograms where pores require TEM or SEM for elucidation).
Callose has been observed, both as a collar surrounding the pores, or as definitive callose.

47. Phloem Phylogeny #3 – where did divergence occur?
MESOZOIC (245)
In cycadophytes such as Cycadeoidea sieve cells alternate with parenchyma
Cretaceous (145)
Sieve elements in vascular cryptograms were considerably different structurally, (perhaps also functionally? From gymnosperms. By the early Carboniferous, these two cell types had diverged.
PALEOZIC, (570)
Sieve cells and sieve tube members (elements) diverge. Seed fern Medullosa sieve cells alternate with parenchyma.
Carboniferous (362)
Slightly oblique end walls in cells occurring in the position which should be occupied by phloem in Rhynia and Trimerophyton.
Middle Devonian (~ 415)

48. So, why two systems?
Even given that the xylem and phloem have distinct functions, with o being confined in an apoplasmic environment, and the other in a symplasmic environment, is this necessary?
One could argue that both functionalities could be contained, maintained and carried out in the same tube – or could one?
It could be argued that one does not ‘need’ a confinement process such as those typified by functional phloem, and functional xylem – or could one?

49. specialization in leaves
Transection of the phloem in a vascular bundle in Beta vulgaris leaf minor vein tissue, showing the narrow-diameter sieve tubes compared with the larger diameter parenchymatous elements.

50. reading
Behnke, H-D and RJ Sjölund (eds) Sieve elements. Comparative structure, induction and Development. Springer ISBN
Carlquist, S Comparative Wood Anatomy. Springer ISBN Car.
Fahn, A 1967 Plant Anatomy. Pergamon Press Fah

51. That’s it……….