Evolution of stele, Heterospory and seed habit
Evolution of stele:-
Stele:- The central cylinder or core of vascular tissue, consisting of xylem, phloem, pericycle and sometimes medullary rays and pith, is technically called stele.
Stelar theory:-
> Van Tieghem and Douliot (1886) developed this theory.
> They used the term stele in collective sense and mentioned that the stele is not only made up of xylem and phloem, but the tissue like pericycle, vascular rays and pith are also associated with it.
> According to them the cortex and the stele are the fundamental parts of a shoot and both these parts are separated from each other by the endodermis.
> According to stellar theory, primarily there is no fundamental difference in the gross anatomy of stem and roots, because in both of them a stele is surrounded by the cortex is present.
> Foster and Gifford (1959) have mentioned that the most debated and controversial aspect of “stellar theory is the nature of the anatomical boundaries which separate the cortex from the stele”.
> According to Van Tieghem and Douliot (1886) the endodermis represents the inner boundary of the cortex. The cells of the endodermal layer have the characteristic casparian strip strips.
> But in the stems of many seed plants, the characteristic endodermal layer is not present. Some have mentioned that in such cases the pericycle serves as separating layer between the stele and the cortex.
Types of stele:- 7 types of steles are found in the whole plant kingdom -
1. Protostele
2. Siphonostele
3. Solenostele
4. Dictyostele
5. Polycyclic stele
6. Eustele
7. Atactostele
Note:- Out of these the first 5 types of steles are found only in the stem of pteridophytes. Eusteel is found in the stem of gymnosperms and dicots. Atactostele is found in the stem of monocots.
Note:- The development of stele has also happened in this sequence. Atactosteel is considered to be the most advanced type of stele.
1. Protostele:- This is the most primitive type of stele in which the pith is completely absent and the phloem surrounds the xylem. According to Jeffrey (1898), the protostele is primitive type of stele in vascular plants.
On the basis of the shape of the xylem in the cross section, it is of 4 types -
i. Haplostele:- The xylem appears circular in cross section. It has been observed in fossil genera like Rhynia, Horneophyton and living genera like Selaginella chrysocaulos, S. kraussiana, S. selaginoides, S. willdenowi, Gleichenia dichotoma, Lygodium and Cheiropleuria.
ii. Actinostele:- The xylem appears star shaped in cross section. Brebner (1902) named such a protostele as actinostele. Eg.- Asteroxylon, Psilotum, Lycopodium serratum and Sphenophyllum.
iii. Plectostele:- In cross section, the xylem appears as parallel plates. Eg.- Lycopodium clavatum and L. volubile.
iv. Mixed protostele:- In cross section, the xylem is seen scattered in the form of segments. Eg.- Lycopodium cernuum
2. Siphonostele:- In this, pith or pith cavity is found in the center around which vascular tissues are found.
Origin of siphonostele:- There are different views among scientists regarding the origin of siphonostele. But they all agreed that the siphonostele originated from protostele by the formation of a pith in the center. Here, the centrally placed xylem core is replaced with a parenchymatous pith.
Different stages of changing protostele to siphonostele can be observed in the T.S. at different levels in Gleichenia, Osmunda, and Anemia.
There are two views regarding the origin of pith in the siphonostele.
i. Jeffrey’s theory:-
- According to this theory, the pith is formed as a result of the invasion of cortical parenchymatous cells into the stele. The invasion of the pith occurs through the leaf gap or branch gap.
- Thus pith and cortex are homogeneous structures according to this theory.
- This theory is not accepted by most of the authors since in many pteridophytes there is a stele without leaf gaps but having siphonostele.
ii. Boodle’s theory:- According to the theory proposed by Boodle (1901), and Gwynne Vaughan, the siphonostele has evolved from the protostele by a transformation of the inner vascular tissue into the parenchyma.
On the basis of the location of phloem it is of two types –
i. Ectophloic siphonostele:- Phloem is found around the xylem. Eg.- Osmunda, Schizaea, etc.
ii. Amphiphloic siphonostele:- Phloem is found at both outside and inside the xylem. Eg. Rhizome of Marsilea, Adiantum, Dipteris.
3. Solenostele:- The vascular tissue in the ectophloic siphonostele forms a leaf gap when it enters the leaf as a leaf trace.
4. Dictyostele:- When the leaves are found densely, many leaf gaps come together causing the vascular tissue of the amphipholic siphonostele to break into many pieces. Each piece is called a meristele and each meristele is a haplostele. In this way many merysteles are found in the form of a circle.
5. Polycyclic stele:- When 2 or more cycles of meristeles are found.
6. Eustele:- When vascular bundles are found in a circle.
7. Atactostele:- When vascular bundles remain scattered throughout the ground tissue.
Heterospory:- The phenomenon of the development of two types of spores (differing in size, structure and function) by the same species is known as heterospory.
> The two different sizes are smaller spores also known as microspores and the larger spores also known as megaspores.
> Rashid (1999) listed nine genera as living examples of heterosporous pteridophytes. e.g. Selaginella, Isoetes, Stylites, Marsilea, Pilularia, Regnellidium, Salvinia, Azolla and Platyzoma.
Origin of Heterospory:- The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies.
1. Palaeobotanical evidences:-
- It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size.
- Palaeobotanical evidences show that the earlier vascular plants were all homosporous and the heterosporous condition appeared subsequently in the lowermost upper Devonian.
- A number of heterosporous genera belonging to the Lycopsida, Sphenopsida and Pteropsida were known in the late Devonian and early Carboniferous periods.
2. Evidences from Experimental Studies:-
- Experimental studies on Selaginella (Goebel, 1905) and Marsilea (Shattuck, 1910) suggest that nutritional factors mainly govern the heterospory.
- Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia.
- By sudden lowering of the temperature, the size of the microspores in the sporocarp of Marsilea increases by six times.
3. Evidences from Developmental Studies:-
- In heterosporous pteridophytes, the development of micro and megasporangia follow the same pattern. While in megasporangia most of the spore mother cells degenerate but in microsporangia only a few mother cells are disorganized.
- The phenomenon of heterospory becomes distinct either before or after meiosis in Selaginella. Isoetes, it is distinct before meiosis.
Importance of heterospory:-
i. The differentiation in the size of the spore is directly related to the differentiation of sex of the gametophyte. Therefore, heterospory is an expression of sex determination in plants.
ii. Because of heterospory, plants become free to grow in varied environmental conditions, instead of only aquatic surroundings.
iii. It ultimately leads to seed development.
iv. The megaspore having female gametophyte derives nourishment from sporophyte and thus remains independent from external factors.
Seed habit:- Because of heterospory, the gametophytic tissue is reduced. Partial or complete retention of megasporangium and female gametophyte on the sporophyte also takes place. They show reduction in megaspores and male gamete. All these characters show a tendency towards seed habit.
Origin of seed habit:- One of the most important advantages of heterospory is the formation of seed habitat. 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.
Seed habit in Selaginella:- Selaginella exhibits remarkable approach to the seed habit because of the following features:
i. The heterospory occurs in almost all the species of Selaginella.
ii. In most species only one functional megaspore mother cell is produced which by reduction division produces four haploid megaspores. In some species e.g. rupestris and S. monospora only one megaspore is formed in each megasporangium.
iii. Germination of megaspore starts within the megasporangium, but its stage and time of retention within megasporangium differ in different species.
iv. Fertilization and embryo development also takes place within the megasporangium.
Although certain species of Selaginella suggest early stages in the evolution of seed habit, Selaginella should not be considered as an ancestor to seed plants because:
i. Absence of integuments around the megasporangia.
ii. Absence of permanent retention of megaspores within megaspoarangia except a few species.
iii. Absence of complete histological fusion between megasporangium wall and megaspore.
iv. After the development of embryo there is lack of resting period.
Therefore, Selaginella has considerably advanced towards seed habit but it’s approach to true seed is not complete.