2 Outlines: Primary Tissues Xylem Tissue Arrangement and Differentiation Stele Evolutionary DevelopmentLeaf Evolutionary Development
3 DifferentiationDevelopmental process by which a relatively unspecialised cell undergoes a progressive change to a more specialised cellI.e. the specialisation of cells and tissues for particular functions during developmentDepends on control of gene expression but determined ultimately by the cell’s final positionCells 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,PROTOXYLEMMETAXYLEM.
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.
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.
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:HaplosteleActinostelePlectosteleSiphonosteleDictyoteleEusteleAtactostele
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 pithFound in most roots and in extinct groups of seedless vascular plantsIf xylem area is circular in crossection it is called a Haplostele (See fig).
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.
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.).
21 The SiphonosteleTwo main kinds of Siphonotele:EctophloicAmphiphloic
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 fernsIn 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.
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
30 B. Protostele (actinostele) A. Protostele(haplostele)B. Protostele (actinostele)C. Protostele (plectostele)E. Siphonostele (amphiphloic)F. DictyosteleD. Siphonostele (ectophloic)Derivation of steles from the primitive protosteleG. EusteleH. 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 traceLeaf Gap:the gap in the ground tissue left behind is called a leaf gapOccur in more advanced siphonostelic or eustelic stems
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 ProtostelesTypical 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 baseEventually extended from the stele into the emergent enation microphyllNo leaf gaps formed with the stele
38 Leaf Development (Megaphyll/Macrophyll): Usually larger than microphyllsAssociated with the siphonosteles of Euphilophytes (which include Ferns and Seed Plants)Evolutionary development (Telome Theory):Began with leafless dichotomously branching plantsUnequal branching then occurred with some becoming more dominant than others (“overtopping”) – pseudomonopodal growthSubordinate / lateral branches leaf via “planation” & “webbing”Associated with leaf gaps