Stele: 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 divided into a number of separate plates which lie parallel to one another e.g. Lycopodium volubile.
(iv) Mixed Protostele: In it xylem and phloem are uniformly distrubted. Xylem elements remain embedded in phloem tissue e.g. Lycopodium cernum the stele is without pith.
(2) Siphonostele or Medulated Protostele:
In it the pith is present in the centre of hollow vascular cylinder. Siphonostele is actually a modified protostele with pith ad it has arisen from intra stellar origin of pith or by invasion of cortical tissue. It is of three types:
(i) Ecophloic Siphonostele: This type of stele has centre pith which is surrounded by concentric rings of xylem and phloem.
(ii) Amploic Siphonostele: In this type of stele the centre is occupied by pith. Xylem is surrounded on both external and inner sides by rings of phloem, pericycle and endodermis e.g. Marsilea.
(iii) Solenostyle: In it the stele is perforated by single leaf. It may be ectopholic or amphipholic Solenostyle in some way as in siphonostele e.g. Ferm.
(3) Dictyostyle:
It is that stele in which there occurs more than one leaf gap. The portion of vascular tissue which lies inbetween two gaps is termed as Meristyle. Each meristyle is protostylic in nature e.g. Ferm.
(4) POLYCYCLIC STELE:
In it the meristeles are distributed in more than one ring and are of different sizes. There occur normally two rings in Pteridium aquillum (Ferm).
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