The Taxonomy and Evolution of Vascular Steles

by: Nate Sammons

Stelar Theory:

Plectostele - Interconnected strands of xylem, in cross section appearing as separate plates of tissue, with phloem occuring between the strands

This diagram presents the extrastelar theory for the evolution of the siphonostele. A. Protostele; B. departure of leaf trace causing an interruption in the vascular cylinder; C. vascular cylinder is closed at arrows; D. siphonostele with C-shaped vascular trace

Adopted from Taylor and Taylor

Dictyostele - modification on the solenostele resulting in two or more overlapping leaf gaps at any given level of the stem

Van Teighen, in response to the growing reliance of vascular organization in morphological interpretation, developed a system which utilized the diversity of transport machinery to form a basis of morphological structure. According to his theory the structure of stems and roots are similar in that each organ consists of a stele, an endodermis, and a surrounding cortex.

?!!!Quiz!!!?

What stele type is this?

Stelar Types:

Ectophloic -

vascular cylinder consisting of xylem surrounded only on the outside by phloem

Protostele -

Vascular tissue characterized by the absence of a central column of pith.

Haplostele - A central strand of primary xylem sheathed by a cylinder of phloem.

Amphiphloic siphonostlele - vascular cylinder consisting of xylem surrounded on both sides by phloem and endodermal layers.

Actinostele - The contour of the core of xylem is lobed or star-shaped in cross-sectional view.

NOTE:

This is considered to be the most primitive of steles.

Siphonostele -

A stele containing a central column of pith

Ectophloic siphonosptele- vascular cylinder consisting of xylem surrounded only on the outside by phloem.

Eustele -

primary vascular cylinder appears in transection as a ring of more or less discrete vascular bundles separated by areas of parenchyma

Atactostele -

consisting of a number of morre or less irregularly arranged vascular bundles together with the ground tissue in which they are imbedded.

This diagram presents the evolution of steles from a primitive protostele. A. Protostele - (Haplostele) B. Protostele (Actinostele) C. Protostele (Plectostele) D. Ectophloic siphonostele E. Amphiphloic siphonostele F. Dictyostele G. Eustele H. Atactostele

Adopted from Practical Anatomy: A.S. Foster

This diagram presents the intrastelar theory for the evolution of the siphonostele. A. Protostele; B. medullated protostele; C. siphonostele with the beginning of a leaf trace; D. siphonostele with C-shaped leaf trace

Adopted from Taylor and Taylor

This diagram presents the evolution of the eustele beginning with a lobed protostele. A. undergoing longitudinal dissection B. resulting in the formation of discrete sympodial bundles C. continued modification through radial divisions of the sympodial bundles (D-E). Results in the formation of a primary vascular system like that of living conifers. (Redrawn from Namboodiri and Beck. 1968c).

Adopted from Taylor and Taylor