Heterospory and Seed habit in Pteridophytes (Ferns)

Evolution of the seed is one of the most crucial adaptations in Phanerogams for successful survival in terrestrial habitat. Seed is the ripened ovule with integuments formed after fertilization.
    Advantages of seed:
    • It offers protection and parental care to the developing embryo.
    • Seed dormancy ensures germination only under favorable conditions, enhancing survival of the new plant.
    •Seed may be defined as a fertilized ovule with protective integuments. ‘Heterospory is the antecedent of seed habit’
    Various steps involved in the evolution of seed habit are as follows:
  1. Evolution of heterospory.
  2. Reduction to a single functional megaspore per sporangium.
  3. Retention and germination of megaspore within the megasporangium.
  4. Development of protective layers and nutritive tissue around megasporangium.
  5. Development of an embryo sac within the sporangium.
  6. Modification of distal end of megasporangium for pollen capture.
  7. Pollination and Siphonogamy
  8. Temporary suspension of growth of embryo (i.e. dormancy period).
    Heterospory and Seed habit
Rounded red circles indicates different steps
In this post we will discuss what all stages Pteridophytes successfully completed in forming a seed?
Step 1: Evolution of heterospory.
• In Pteridophytes , majority are homosporous, but Selaginella, Marselia, Salvinia, Azolla etc are heterosporus showing tendency towards seed habit. Land plants up to pteridophytes are homosporous. The next big step in evolution was heterospory or formation of two different types of spores, the smaller ones called microspores and the larger ones known as megaspores.
• Microspores are formed inside microspornagia which on germination form male gametophyte bearing antheridium while megaspores are formed inside megasporangium and give rise to female gametophyte with archegonium
Step 2: Reduction to a single functional megaspore per sporangium.
• In Selaginella rupestris and S.monospora, only 4 megaspores are produced out of which only one become functional.
• In higher plants, each megaspore mother cell within a megasporangium undergo meiosis forming four megaspores. Of the four, only one is retained ensuring enough space, food and greater chance of developing into a healthy female gametophyte.
Step 3: Retention and germination of megaspore within the megasporangium.
• In Pteridophytes and other lower groups, the megaspores are used to be shed and dispersed soon after their formation in order to germinate into female gametophyte.
• In S. rupestris, only after embryo formation, megagametophye is shed. Hence S.rupestrisis is very near to seed bearing plants as it retains megaspore till embryo formation in megasporangium for a considerable period. In angiosperms, it is retained.
Step 4: Development of protective layers and nutritive tissue around megasporangium.
• In angiosperms, formation of integuments and nucellus offers protection and nourishment to the developing embryo.
• In pteridophytes, no specialized nutritive tissues or integuments are formed. But In Selaginella, nutritive tissue called tapetum is present. Many spore mother cells disintegrate to form nutritive tissue. Megaspore is protected by its wall also but not by an integument.
Step 5, 6, 7, 8:
5)Development of an embryo sac within the sporangium.
6)Modification of distal end of megasporangium for pollen capture.
7) Pollination and Siphonogamy.
8)Temporary suspension of growth of embryo (i.e. dormancy period).
In pteridophytes, further steps towards seed habit are lacking. There is no embryo sac formation.
No siphonogamy, therefore water is essential for fertilization
Seed habit in SelaginellaIn the figure, Solid red circle indicates : successful, dotted circle: to some extend
This summarize the successful steps of Pteridophytes towards seed habit.
ü 1. Evolution of heterospory: successful
ü 2. Reduction to a single functional megaspore per sporangium: Successful in some species
ü 3. Retention and germination of megaspore within the megasporangium (to some extend): Successful to some extend
ü 4. Development of protective layers and nutritive tissue around megasporangium (to some extend): Successful to some extend
ᵪ 5. Development of an embryo sac within the sporangium.
ᵪ 6. Modification of distal end of megasporangium for pollen capture.
ᵪ 7. Pollination and Siphonogamy
ᵪ 8. Temporary suspension of growth of embryo (i.e. dormancy period).
Among Pteridophytes, Selaginella rupestris is the most successful species that completed first four steps towards seed habit.
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