Gymnosperms

Gymnosperms

For the lower vascular shews the important evolutionary training was in the water and food conducting weaves of the sporophyte. As we move on finished the plant kingdom the next important development was the spill. The ease living gametophyte is a vulnerable phase of the life cycle. counterpart by seeds is a less chancy procedure and has another(prenominal) advantages for plant survival and dispersal. Seeds can be remarkably genial of environmental extremes heat, cold and drought. Unlike free-living gametophytes seeds can turn off their development until conditions be right. And, of course, we find them very convenient for plant propagation.1

Already in the coal-measure forests there were plants that reproduced by seeds. Some were the so-called seed ferns. none of which survive. Others were the ancestors of the plants we now know collectively as gymnosperms. In these plants the seeds are not enclosed in an ovary, as in the flowering plants; they grow on the surface of a circumscribed leaf in a strobilus or cone. Gymnosperm nub naked seed.1

Alternation of generations is still involved in the reproduction of these plants. They are all heterosporous: the microspores are shed as pollen, whereas the megaspore germinates in the strobilus to produce the female gametophyte.



The archegonia in this gametophyte work fertilized by sperm from the male gametophyte and the zygote grows to produce an fertilized egg which is enclosed in a seed coat of tissue from the parent plant.1

Gymnosperms were the dominant land plants in the age of dinosaurs, the cretaceous and Jurassic periods. The surviving gymnosperms in the Coniferophyta, Cycadophyta and Ginkgophyta are uniform in their woody habit and pattern of seed development but are not closely related.1

Some gymnosperms are among the tallest, largest and oldest organisms in the world. In New Zealand, rimu (Dacrydium cupressinum) and kahikatea (Dacrycarpus...

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