Heterospory and Seed habit in Selaginella

In the last post, we discussed in detail the tendency of Pteridophytes towards seed habit. In this post, we will be discussing only major points regarding Selaginella’s strides towards seed habit.

These are the basic requirements for seed habit:

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).

This summarize the successful steps by genus Selaginella towards seed habit

S.rupestris is very close to seed habit as it exhibits

In the figure, Solid red circle indicates : successful, dotted circle: to some extend

1. Evolution of heterospory: In all Selaginella species

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.

3. Retention and germination of megaspore within the megasporangium (to some extend): In S. kraussiana, the female gametophyte develops in situ and it even produces archegonia. In S. rupestris, only after embryo formation, megagametophye is shed.

4. Development of protective layers and nutritive tissue around megasporangium (to some extend): 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.

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 one species that is very close to seed habit.