Differentiation in Seed Plants

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

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

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

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.

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.

Protoxylem:

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.

Metaxylem: Pits

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

MESARCH EXARCH ENDARCH CENTRARCH

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

Types of stele

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).

Protostele

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.

Protostele

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.).

Plectostele

The Siphonostele Two main kinds of Siphonotele: Ectophloic Amphiphloic

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.

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

Cortex Siphonostele

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.

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

Eustele

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

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

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

Vascular Arrangements:

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.

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

Microphyll Development

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

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