IGNOU LSE 12 2015 - Discuss the stellar structure and its evolution in Pteridophytes.

Question

Discuss the stellar structure and its evolution in Pteridophytes.

07 Feb 2018

Answer :

Word Count : 769

Word stele is taken from Greek which means Pillar. According to vantighem and Douliot in 1886 put forward “steler theory” the stele is the whole central mass of vascular tissue with or without pith surrounded on the outer side by endodermis. Endodermis is either the part of setle or cortex is controversial. Thus steel consists of a layer of pericycle, xylem, phloem and pith (if present).

Types of stele in pteridophytes:

(1) Protostele

(2) Siphonostele

(3) Solenostele

(4) Dictyostele

(5) Polycyclic stele

(1) PROTOSTELE:

According to Jeffry in 1903 Prostele is primitive and simple type of stele. It consists of solid amss of xylem surrounded by phloem, pericycle and endodermis. It is permanently present in adult stems of many living pteridophytes e.g. Lygodium, Lycopodium, Hymemophyllus and Selaginalla. It is present in vascular plants. Protostele is of four types as:

(i) Haplostele: Haplostele is that in which the central xylem has smooth outline and surrounded by concentric layers of phloem e.g. Lygodium, Selaninella and sps. Of Lycopodium.

(ii) Actinostele: In it central xylem cone has radiating ribs and assumes a star shaped appearanace e.g. Lycopdoum, Serratum, Psilotium.

(iii) Plectostele: In it xylem is

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