Stelar Evolution in Vascular Plants
(Origin and Evolution of Stele in Pteridophytes and Higher Plants)
What is stele? What are the components of stele?
Ø Stele is the central cylinder or core of vascular tissue in higher plants.
Ø The term ‘stele’ was for the first time used by Van Tieghem and Douliot in 1886 in their ‘Stelar Theory’.
What is ‘stellar theory’?
Ø Proposed by Van Tieghem and Douliot in 1886.
Ø Major highlights in stellar theory are:
$. The stele is a real entity and present universally in all axis of higher plants.
$. The primary components of stele are xylem and phloem.
$. Tissues like pericycle, medullary rays and pith are also the components of stele.
$. ‘Stelar theory’ also says that the cortex and the stele are the two fundamental parts of a shoot system.
$. Both these components (stele and cortex) are separated by the endodermis.
$. In higher vascular plants (Pteridophytes, Gymnosperms and Angiosperms), the leaf traces are large, and it appears that they play an important role in the vascular system of the axis.
$. The whole set-up of leaf traces appears as a composite structure in these plants.
$. Such composite structures do not remain within the limits of stellar theory of Van Tieghem and Douliot.
What are the different types of steles in plants (Pteridophytes and higher plants)?
Ø On the basis of ontogeny and phylogeney, there are THREE broad categories of steles in vascular plants.
Ø They are: