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PinkMonkey Online Study Guide-Biology

ii) Siphonostele (Fig 15.25B): It has evolved from the protostele. In siphonostele, pith is present in the center of the xylem cylinder. The phloem is external to xylem.

iii) Solenostele (Fig. 15.25): A Cbroken at one point at a time due to the non-overlapping leaf gaps is called solenostele

iv) Dictyostele (Fig. 15.25D): Siphonostele in which many overlapping leaf gaps occur at the same level is called a dictyostele, or dissected siphonostele. In a transverse section it appears broken into many smaller or larger arcs. Each arc is known as a merestele (e.g. fern rhizome). Each meristele is completely and independently surrounded by the pericycle and the endodermis. This is the most highly- evolved stele type in Pteridophyta.

v). Eustele (Fig. 15.25E): The characteristic stele present in dicot and gymnosperm stems is called a eustele (i.e." true" stele). It is also a dissected siphonostele in which the vascular cylinder is dissected into separate vascular bundles arranged in a ring. The entire ring of vascular bundles is surrounded by the pericycle and the endodermis.


v) Atactostele ( Fig. 15.25F): This is a complex type of stele in which numerous vascular bundles remain irregularly scattered in the ground tissue (atactos means without any order). Atactostele is a characteristic of monocot stems. Here, due to the scattered vascular bundles, the cortex and stele are not demarcated. Similarly, the endodermis and the pericycle are not differentiated.

SUMMARY- VASCULARIZATION

Conducting tissues in sporophyte plant body of vascular plants were evolved in response to terrestrial habitat. Xylem conducts water and minerals, while phloem conducts organic solutes. The strands of xylem and phloem form vascular bundles, which may or may not have cambium. The vascular bundles are organized into various types of stele. The simplest type is the protostele, while the eustele of dicots and the atactostele of monocots are supposed to be the most evolutionary advanced type.

Table of Contents

15.0 - Introduction
15.1 Pteridophyta : General Account
15.2 Gymnosperms
15.3 Angiosperms : Dicotyledons
15.4 Angiosperms : Monocotyledons
15.5 Vascularization
15.6 Development of seed habit
15.7 Development of Flower and Fruit

Chapter 16





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