1. Differentiation in Seed Plants

2. Outlines: Primary Tissues Xylem Tissue Arrangement and Differentiation
Stele Evolutionary Development
Leaf Evolutionary Development

3. Differentiation
Developmental process by which a relatively unspecialised cell undergoes a progressive change to a more specialised cell
I.e. the specialisation of cells and tissues for particular functions during development
Depends on control of gene expression but determined ultimately by the cell’s final position
Cells capable of communicating positional information from one cell to another

4. Embryo Differentiation:
Much variation in the patterns of development observed particularly within the vasculature

6. Xylem Arrangement:
With respect to the pattern or direction of differentiation of primary xylem from the PROCAMBIUM, 4 relationships are known.
However, their recognition depends on a distinction between two kinds of PRIMARY XYLEM,
PROTOXYLEM
METAXYLEM.

7. Protoxylem: the first-formed primary xylem,
its elements usually smaller in diameter,
differentiates before elongation of the organ has ceased,
and lignified in annular or spiral fashion.

8. Protoxylem:

9. Metaxylem: differentiates after the PROTOXYLEM,
its elements are larger in diameter,
they mature only after elongation of the organ has ceased,
and their secondary wall pattern is usually reticulate or pitted.

10. Metaxylem:
Pits

11. Xylem Differentiation:
However, there are transitions between protoxylem and metaxylem
The four patterns of primary xylem differentiation from the procambium are:
EXARCH
MESARCH
ENDARCH
CENTRARCH

12. MESARCH
EXARCH
ENDARCH
CENTRARCH

13. STELES
(Greek – “pillar”) The central cylinder, inside the cortex, of roots and stems of vascular plants.
The term stele sums up the vascular system, associated tissues and the enclosed pith.
General types of stele that evolved over time:
Protostele:
Haplostele
Actinostele
Plectostele
Siphonostele
Dictyotele
Eustele
Atactostele

14. Types of stele

15. Protostele: Haplostele
Protosteles are the simplest and are often considered the most primitive type of stele.
It is essentially a solid core of xylem surrounded by a cylinder of phloem, a protostele contains no pith
Found in most roots and in extinct groups of seedless vascular plants
If xylem area is circular in crossection it is called a Haplostele (See fig).

16. Protostele

17. Protostele: Actinostele
A protostele with radiating arms.
The central xylem instead of being round becomes a radiated star shaped arm with the protoxylem being found in the arms and the metaxylem being found central.

18. Protostele

19. Protostele: Plectostele
Is a protostele in which phloem is interspersed in masses between the xylem.
That is, the radiating arms develop more into a plate like structure.
The xylem although interrupted at a given level, forms a continuous system (see fig.).

20. Plectostele

21. The Siphonostele
Two main kinds of Siphonotele:
Ectophloic
Amphiphloic

22. The Siphonostele: Ectophloic
The siphonostele is widely distributed, occurring in ferns and in certain gymnosperms and flowering plants.
In the siphonostele the xylem and the phloem forms a cylinder around a central pith with no gaps.
The phloem may be both external and internal as it is in many ferns
In ectophloic siphonosteles the phloem is restricted to the outer surface of the xylem.

23. Siphonostele: Amphiphloic
In this case it is said to be amphiphloic siphonostele, and is sometimes called a solenostele.

24. Cortex
Siphonostele

25. Siphonostele: Dictyostele
In a solenostelic stem with short internodes, overlapping of the nodal leaf gaps results in a dissection of the stele (See fig).
That is, there are one or two leaf gaps in the xylem and phloem created by leaf traces.
In crossection the stele appears as discrete strands or bundles.

26. Siphonostele: Eustele
Where the xylem and phloem occur in discrete collateral (situated side by side) or bicollateral strands or bundles; the arrangement is called a Eustele

27. Eustele

28. Siphonostele: Atactostele
Here the discrete strands of xylem and phloem are scattered through the stem
This is the arrangement found in monocotyledonous flowering plants.

30. B. Protostele (actinostele)
A. Protostele(haplostele)
B. Protostele (actinostele)
C. Protostele (plectostele)
E. Siphonostele (amphiphloic)
F. Dictyostele
D. Siphonostele (ectophloic)
Derivation of steles from the primitive protostele
G. Eustele
H. Atactostele

31. Leaf development: Leaf Trace: Leaf Gap:
A vascular bundle branch that extends from a stem bundle to the base of a leaf, where it connects with the vascular system of the leaf, is called a leaf trace
Leaf Gap:
the gap in the ground tissue left behind is called a leaf gap
Occur in more advanced siphonostelic or eustelic stems

32. Vascular Arrangements:

33. Leaf Types:
There are two types of leaves depending on the type of leaf trace:
Microphyll: A leaf arising out of a leaf trace without any gaps.
Megaphyll: A leaf arising out of a leaf trace which has a leaf gap.
In the microphyll the stele of the stem is protostellic with no gaps between the leaf traces.
In the megaphyll the leaf trace can leave a gap which is filled by parenchymatous tissue. Usually siphonostellic.

36. Leaf Development (Microphyll):
Microphylls: relatively small leaves with a single vascular strand, usually associated with Protosteles
Typical of the seedless vascular plant group – Lycophytes (club mosses)
Evolutionary development:
Began as scale-like outgrowths with no vascular tissue (Enation)
Leaf trace then formed, initially at base
Eventually extended from the stele into the emergent enation  microphyll
No leaf gaps formed with the stele

37. Microphyll Development

38. Leaf Development (Megaphyll/Macrophyll):
Usually larger than microphylls
Associated with the siphonosteles of Euphilophytes (which include Ferns and Seed Plants)
Evolutionary development (Telome Theory):
Began with leafless dichotomously branching plants
Unequal branching then occurred with some becoming more dominant than others (“overtopping”) – pseudomonopodal growth
Subordinate / lateral branches  leaf via “planation” & “webbing”
Associated with leaf gaps

40. Recap… Three 1o tissues derived from 3 1o cambia
Protoderm  Epidermal Tissues
Ground Meristem  Ground Tissues
Procambium  Vascular Tissues
Two main kinds of Stele:
Protostele (primitive)  Siphonostele (more advanced)
Atactostele – most advanced
Leaf development
Primative microphyll
Advanced megaphyll