There is a gradual change in the structure of seeds in plants any Selaginella two types of spores are produced, microspores, smaller in size and megaspores, large in size. This process is called heterospory. The microspores develop into male gametophytes and megasapores develop into female gametophyte. This is the process of “Seed Evolution”. In carboniferous era some ferns like plants were produced and their sporangia were surrounded by protective covering.
In seed plants there is different condition. Unlike other green plants, in seed pants the megaspores remain inside the sporangia, which are covered by protective covering called integuments. The megaspores develop into female gametophyte, which is also protected by integuments.
The seed plants are dominant vascular plants. All seed producing plants are called spermatophytes. The development of seed habit is the most important step in the history of land plants. It has changed the reproductive system of the vascular plants. The evolution of seed occurred about 365 million years ago during late Devonian period.
Seed may be defined as “a fertilized ovule.” An integumented and indehiscent megasporangium is called ovule. The integuments are specialized protective covering around the megasporangium. The number of integuments is different in different plants.
Following steps took place in the seed evolution:
- Evolution of Heterospory.
- Retention of germination of megaspore within the Megasporngium.
- Development of protective layers around megasporangium.
- Reduction to a single functional megaspore per sporangium.
- Development of an embryo sac within the sporangium. .
- Modification of distal end of megasporangium for capturing pollen grains.
1. Evolution of heterospore
Primitive vascular plants produce one kind of spores. This condition is called heterospory. All the groups of the land plants up to pteridophytes are homosporous. Some plants started producing two types of spores during early phase of evolution. These spores are:
- Microspores: They are smaller spores. They are produced inside the microsporangia. Microspore germinates to form male gametophyte or microgametophyte.
- Megaspores: They are larger spores. They are produced inside the megasporangium. They germinate to form female gametophyte or Megagametophyte.
2. Retention and germination of megaspore within the megasporangium
The megaspore is discharged from megasporangium in early heterosporous vascular plants. This megaspore is dispersed and germinates to form female gametophyte. However, in some plants like Selaginella, the megaspores are not discharged from the megasporangium. The megaspores are permanently retained within the megasporangium in such plants. This megaspore germinates within the megasporangium to form female gametophyte with its egg.
3. Development of protective layers around the megasporangium
Some branch like structures of sporophyte grow and surround the megasporangium. These branches fuse with the megasporangium to form protective envelope or integument. The megasporangium is tightly locked by this integument. It becomes totally indehiscent. This important change caused the evolution and formation of ovule. Ovule is an integumented indehiscent megasporangium. In this way, it gives more protection to egg containing apparatus (ovule) in the terrestrial environment.
4. Reduction to a single functional megaspore per sporangium
Each megaspore mother cell divided inside within the megasporangium by meiosis to form four megaspores. These megaspores germinated to produce four viable female gametophytes. There was competition among the four gametophytes for space and food. Later, there was a new adaptation in the early vascular plants. They selected only one megaspore for further development. The remaining three megaspores were aborted .This megaspore formed a healthy female gametophyte.
5. Development of an embryo sac within the sporangium
The single megaspore was retained within the megasporangium. It germinates to form egg containing female gametophyte called embryo sac.
6. Modification of distal end of megasporangium for pollen capture
Most of the structural and functional changes for the seed evolution have completed. Now megasporangium is integumented, indehiscent and permanently attached to sporophyte. Another important modification took place in the megasporangium. The distal end of the megasporangium was modified for capturing pollen. Pollen (singular: pollen grain) is a microspore containing male gametophyte.
The pollen is trapped in the distal cavity of the megasporangium. This pollen produces pollen tube. Pollen tube carries male gamete deep into the embryo sac and fertilization takes place and zygote is formed. This zygote develops to form embryo. After fertilization, the megasporangium (ovule) is transferred into a seed. The integument of ovule forms the seed coat. The seed protects the developing embryo from unfavorable conditions in terrestrial environment. The evolution and development of seed was a great success. It ultimately enabled the plants to grow on land permanently.