There is a gradual change in the structure of seeds in plants Selaginella’s 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 megaspores develop into female gametophytes. This is the process of “Seed Evolution”. In the carboniferous era, some ferns like plants were produced and their sporangia were surrounded by a protective covering.
In seed plants, there is a different conditions. Unlike other green plants, in seed plants, the megaspores remain inside the sporangia, which are covered by a protective covering called integuments. The megaspores develop into a 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 vascular plants. The evolution of seed occurred about 365 million years ago during the Late Devonian period.
A seed may be defined as “a fertilized ovule.” An integument and indehiscent megasporangium is called an ovule. The integuments are specialized protective covering around the megasporangium. The number of integuments is different in different plants.
The 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 heterosporous
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 the 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 gametophytes or Megagametophytes.
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 a 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 a 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 a 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 the ovule. The ovule is an integument indehiscent megasporangium. In this way, it gives more protection to the egg-containing apparatus (ovule) in the terrestrial environment.
4. Reduction to a single functional megaspore per sporangium
Each megaspore mother cell is 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 an egg-containing female gametophyte called an embryo sac.
6. Modification of distal end of megasporangium for pollen capture
Most of the structural and functional changes for seed evolution have been completed. Now megasporangium is integument, indehiscent, and permanently attached to the 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 a pollen tube. The pollen tube carries the male gamete deep into the embryo sac and fertilization takes place and the zygote is formed. This zygote develops to form an embryo. After fertilization, the megasporangium (ovule) is transferred into a seed.
The integument of the ovule forms the seed coat. The seed protects the developing embryo from unfavorable conditions in the terrestrial environment. The evolution and development of seed was a great success. It ultimately enabled the plants to grow on land permanently.