Stele The term stele has been derived from a Greek word meaning pillar Stele is the central vascular cylinder with or without pith and delimited by endodermis

TYPES OF STELE c) Dictyostele a) Protosteles d) Eustele b) Siphonostele e) Atactostele

Apmhiphloic siphonostele Protostele Actinostele Plectostele Siphonostele Solenostele Eustele Apmhiphloic siphonostele Dictyostele Attactostele

Types of Stele The stele may be of following types: 1. Protostele : The primitive type stele is protostele, here the vascular tissue is a solid mass and the central core of xylem is completely surrounded by the strand of phloem.

Types of protostele a) Haplostele: This is the most primitive type of protostele. Here the central solid smoth core of xylem remains surrounded by phloem (e.g. Selaginella spp.). b) Actinostele: this is the modification of the haplostele and somewhat more advanced in having the central xylem core with radiating ribs (e.g. Psilotom spp.).

c) Plectostele: this is the most advanced type of protostele c) Plectostele: this is the most advanced type of protostele. Here the central core of xylem is divided into a number of plates arranged parallel to each other. The phloem alternates the xylem (e.g. Lycopodium spp.). d) Mixed-pith stele: Here the xylem elements(tracheids) are mixed with parenchymatous cells of the pith. They are treated to be the transitional types in between true protostele and the siphonostele

Haplostele

Actinostele : A *protostele in which the central core of xylem is star shaped or somewhat lobed as viewed in transverse section. This type of stele is exhibited in various species of Psilotum and Lycopo-dium and in the roots of higher plants. Depending on the number of xylem lobes, usually between two and eight, the stele is described as diarch, triarch, etc., and polyarch if the number of lobes exceeds eight.

PLECTOSTELE

2. Siphonostele Here, pith is present and surrounded by vascular bundle. The siphonostele may be following types: a) Ectophloic : In this type of siphonostele, the pith is surrounded by concentric xylem cylinder and next to xylem the concentric phloem cylinder.

b) Amphiphloic The pith is surrounded by the vascular tissue b) Amphiphloic The pith is surrounded by the vascular tissue. The concentric inner phloem cylinder surrounds the central pith. Next to the inner phloem is the concentric xylem cylinder which is immediately surrounded by the outer phloem cylinder (e.g. in Marsilea).

3. Dictyostele It is the advanced type of siphonostele where leaf gaps are present and the successive gaps may be overlap each other.

4. Eustele stele Here the vascular system consists of a ring of collateral or bicollateral vascular bundles situated on the periphery of the pith. In such steles, the interfascicular areas and the leaf gaps are not distinguishable each other very clearly.

Atactostele In Atactostele consists of collateral bundles distributed in more that one ring within the Stem. This does not mean that the bundles are randomly arranged. A complex organizational pattern is present, but it is not obvious to a casual observer. A Pith is not present. There is no Endodermis. This stele is typical for Monocots like Sugarcane.

Siphonostele Plants that produce complex leaves also produce more complex steles. The hormones produced by the young leaf and its associated axillary bud affect the development of tissues within the stele producing leaf gaps. These plants have a pith in the center of their stems, surrounded by a cylinder containing the vascular tissue. This stelar arrangement is termed a siphonostele. There are three basic types of siphonostele:

Atactostele: Stele in which primary vascular bundles are scattered or spirally arranged throughout