Gymnosperms is that sub-division of Spermatophyta which includes those plants which bear naked seeds i.e., the ovules and the seeds that develop from these ovules are not enclosed in fruit wall. In other words the ovary is absent.
In comparison to angiosperms it is a smaller group having only 700 species. Even then they are distributed throughout the whole world. They are dominant vegetation in some forests. They are more advanced than bryophyta and pteridophyta. Many gymnosperms are fossils.
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Important Characteristics
1. Most plants are perennial and woody.
2. They are xerophytic, with sunken stomata and thick cuticle.
3. The xylem is without vessels and phloem lacks companion cells.
4. Reproductive organs are usually in the form of cones or strobili. The male cones are made up of microsporophylls and female cones are made up of megasporophylls.
5. Microsporophylls bear microsporangia (900-1000 microsporangia in sori in Cycas and two microsporangia in Pinus sps.).
6. Large number of microspores (pollen grains), are produced in microsporangia after reduction division, which later form male gametophytes.
7. Male gametophytes of all gymnosperms produces pollen tubes.
8. In the nucellus of megasporangia (ovules) one of the cells forms megaspore mother cell which after reduction division forms a linear tetrad of megaspores of which only one is functional.
9. Functional megaspore forms female gametophyte (also called endosperm). Thus the endosperm is a pre-fertilization tissue in gymnosperms).
10. On the micropylar end of female gametophyte are differentiated two or more archegonia.
11. All gymnosperms are wind pollinated.
12. Usually polyembryony is found in seed although ultimately only one embryo survives.
13. Archegonia lack neck canal cells.
14. Embryo bears two as in Cycas or more (10-11 in Pinus) cotyledons.