Unit V: Pteridophyte

The term stele refers to the central cylinder of vascular tissues consisting of xylem, phloem, pericycle and sometimes medullary rays with pith.

There are two types of steles

1. Protostele

2. Siphonostele

1. Protostele:

In protostele phloem surrounds xylem. The type includes Haplostele, Actinostele, Plectostele, and Mixed protostele.

(i) Haplostele: Xylem surrounded by phloem is known as haplostele. Example: Selaginella.

(ii) Actinostele: Star shaped xylem core is surrounded by phloem is known as actinostele. Example: Lycopodium serratum.

(iii) Plectostele: Xylem plates alternates with phloem plates. Example: Lycopodium clavatum.

(iv) Mixed prototostele: Xylem groups uniformly scattered in the phloem. Example: Lycopodium cernuum.

2. Siphonostele:

In siphonostele xylem is surrounded by phloem with pith at the centre. It includes Ectophloic siphonostele, Amphiphloic ­siphonostele, Solenostele,

(i) Ectophloic siphonostele: The phloem is restricted only on the external side of the xylem. Pith is in centre. Example: Osmunda­.

(ii) Amphiphloic siphonostele: The phloem is present on both the sides of xylem. The pith is in the centre. Example: Marsilea.

(iii) Solenostele: The stele is perforated at a place or places corresponding the origin of the leaf trace.

(a) Ectophloic solenostele – Pith is in the centre and the xylem is surrounded by phloem Example Osmunda.

(b) Amphiphloic solenostele – Pith is in the centre and the phloem is present on both sides of the xylem. Example: Adiantum­ pedatum.

(c) Dictyostele – The stele is separated into several vascular strands and each one is called meristele. Example: ­Adiantum capillus-veneris.

(iv) Eustele: The stele is split into distinct collateral vascular bundles around the pith. Example: Dicot stem.

(v) Atactostele: The stele is split into distinct collateral vascular bundles and are scattered in the ground tissue Example: Monocot stem.

(vi) Polycyclicstele: The vascular tissues are present in the form of two or more concentric cylinders. Example: Pteridium.

Heterospory in Pteridophytes:

Most of the Pteridophytes produce one kind of similar spore. Such Peridophytes are known as homosporous and this phenomenon is known as homospory. However, there are some Pteridophytes which produce two different types of spores (differing in size, structure and function).

Such Pteridophytes are known as heterosporous and the phenomenon is known as heterospory. The two types of spores are microspores and megaspores. Microspores are smaller in size and develop into the male gametophyte while the megaspores are large and develop into female gametophyte.

According to Rashid (1976) only 9 genera of Pteridophytes are heterosporous. These are Selaginella, Isoetes, Stylites, Marsilea, Pilularia, Regnellidium, Salvinia, Azoll and Platyzoma.

Biological Significance of Heterospory:

(i) The development of the female gametophyte starts while the megaspore is still inside the megasporangium.

(ii) Same is true of microspores i.e., they also start germinating into male gametophytes while they are still inside microsporangium.

(iii) The female gametophyte derives its nourishment from the sporophyte i.e., female gametophyte is dependent on sporophyte for its nourishment.

(iv) The dependence of female gametophyte on sporophyte for its nourishment provides better starting point for the development of new embryo than an independent green prothallus which has to manufacture its own food.

Seed Habit in Pteridophytes:

The adoption of heterospory and the retention and germination of a single megaspore within megasporangium to form a female gametophyte, led to the phenomenon of “seed habit”, a characteristic feature of the spermatophytes. A seed is that ovule which contains an embryo developed as a result of fertilization.

The origin of seed habit is associated with the following:

(i) Production of two types of spores (heterospory).

(ii) Reduction in the number of megaspores finally to one per megasporangium.

(iii) Retention and germination of the megaspores and fertilization of the egg.

(iv) Continued development of the fertilized egg into the embryo while still in situ.

Selaginella approaches towards seed habit because of the following features:

1. The occurrence of the phenomenon of heterospory.

2. Germination of megaspore inside megasporangium.

3. Retention of megaspore inside megasporangium either till the formation of female gametophyte or even after fertilization.

4. Development of only one megaspore per megasporangium for example, in Selaginella monospora, S. rupestris, S. erythropus etc.