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In
the previous laboratory we studied two groups
of terrestrial plants; the bryophytes
and the seedless vascular plants. The second major group of vascular plants, the seed plants, has been by far the most successful in exploiting the terrestrial environment.
There are three main reasons for their success:
Seed plants are heterosporous, producing female spores (megaspores) and male spores (microspores).
During early development of the female ovule (unfertilized seed), four megaspores are produced by meiotic division of the megasporocyte cell (megaspore mother cell) in the megasporangium (nucellus) of the ovule. The megasporangium is non-dehiscent; thus, the megaspores are retained in the ovule and never released from the megasporangium. Generally, three of the four megaspores degenerate. Within the cytoplasm of the surviving functional megaspore (endosporic development), a microscopic, multicellular megagametophyte (female gametophyte) develops by mitosis. One cell of this gametophyte phase will function as an egg. Concurrent with these events, a protective envelope of tissue, the integument , develops around the megasporangium of the ovule. A narrow canal in this envelope, called the micropyle, provides an open pathway to the internal megasporangium and megagametophyte. It is the integument which develops into the protective seed coat of the mature seed.
Four microspores are produced by meiotic division of a microsporocyte cell (pollen mother cell) in the microsporangium . There are numerous microsporocyte cells within each microsporangium resulting in the production of many thousands of microspores. Within the cytoplasm of each microspore (endosporic development), a microscopic, multicellular microgametophyte (male gametophyte) develops by mitosis. The entire structure is commonly called a pollen grain. The pollen tube is a unicellular outgrowth of the inner wall of the pollen grain and it is this tube which transports the two sperm cells of the microgametophyte to the egg cell of the megagametophyte in the ovule.
The seed plants have traditionally been divided into two classes, the Gymnosperms and the Angiosperms (flowering plants). The name gymnosperm means "naked seed" and implies that the seeds are developed in an exposed position on the parent sporophyte (often on the upper surface of a female cone scale). This condition contrasts with that of the angiosperms in which the seed develops within and is enclosed by a portion of the parent sporophyte flower, the carpel (e.g. a pea pod or a segment of an orange).
The largest and most important group of gymnosperms are the conifers. Some of the largest vascular plants, such as the redwood (Sequoia), and the oldest living plants such as the Bristlecone Pine (over 4,000 years old) belong to this group.
