Summary: Trends of evolution via evolution of theSeed/Pollen habit

I. Trends in reduction of size and complexity - i.e. increased specialization

A. Gymnosperms

Female
1. Only one megaspore mother cell develops in each megasporangium
2. Few archegonia develop

Male
3. The microgametophyte is few-celled
4. There is no antheridium
5. There is only one functional sperm

B. Angiosperms

Female
6. The megagametophyte consists of only 7 cells
7. There is no archegonium

Male
8. The development of the microgametophyte is restricted to only 2 mitotic events

II. Trends in increased protection of the vulnerable reproductive structures

A. Gymnosperms

Female
1. The vascularized integuments protects (encloses) the entire ovule/seed
2. The megasporangium encloses the megaspore, and later the megagametophyte
3. The megagametophyte encloses the zygote, and later the embryo

Male
4. The pollen wall protects (encloses) the entire microgametophyte

B. Angiosperms

Female
5. The ovule/seed possesses a double layered integument
6. The ovules/seeds are borne in an ovary

III. Trends in increased efficiency in nutrition of reproductive parts

A. Gymnosperms

1. The megagametophyte digests the megasporangium
2. The embryo digests the megagametophyte

B. Angiosperms

3. The embryo is nourished by the 3n endosperm

IV. Increased dependence of the gametophyte on the sporophyte

The entire 1n phase of the life cycle plus sporangium and embryo is attached to and dependent on the sporophyte

QUESTIONS TO CONSIDER

1. What has been the general trend in the evolution of land plants in regard to the duration of and structure of the sporophyte and gametophyte stages?

2. What has been the general trend in the evolution of land plants in regard to the exposure of gametes to the environment?

3. What condition of the environment is necessary for fertilization in all lower vascular plants but not in conifers or flowering plants?

4. Describe where meiosis occurs in a moss, fern, pine, and lily. Be specific.

5. Describe where fertilization occurs in a moss, fern, pine, and lily. Again, be specific.

6. For both meiosis and fertilization, what are the similarities and differences among these groups?