15.5 Vascularization

Evolution of vascular tissues in plants was one of the essential adaptations to terrestrial habitats. Vascular tissues are the specialized conducting tissues, xylem and phloem. Plants with vascular tissues are grouped as the Tracheophyta and include Pteridophyta, Gymnosperms and Angiosperms.

Figure 15.24

Xylem: It is a specialized tissue for conduction of water and minerals from roots to all other parts of the plant. Xylem is a complex and heterogeneous tissue composed of four types of elements namely (a) tracheids ,(b) vessels ,(c) fibers ,and (d) xylem parenchyma (Fig. 15.24A). The first three elements are dead and lignified. Hence, xylem is described as a dead tissue.

Phloem: It is a living, complex and heterogenous food-conducting tissue. It is composed of (a) sieve tubes, (b)companion cells, (c) phloem fibers and (d) parenchyma (Fig. 15.24B).

In plants, vascular tissues are organized into strands called vascular bundles. These are spread over all parts of the plant and form the vascular tissue system. In gymnosperms and dicotyledons, cambium is also present in the vascular bundles.

Steles: The vascular tissues are organized into steles. The vascular bundles, with or without other non- vascular tissues, when surrounded by the pericycle and the endodermis are referred to as steles.

Main types of steles and their evolution

i) Protostele (Fig 15.25A): This is the simplest and most primitive type of stele in which xylem forms the central cylinder and is completely surrounded by phloem cells. Pith is totally absent. Common in pteridophytes like Psilotum, Lycopodium, etc.

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