CHAPTER XI 



CYTOLOGY OF REPRODUCTION IN PLANTS OTHER 

 THAN ANGIOSPERMS 



In nearly all plants of the groups now to be considered — gymnosperms, 

 ferns, mosses, liverworts, algae, and fungi— the life cycle, like that of 

 angiosperms, includes two kinds of reproductive cells: gametes and 

 spores. Diplosis occurs at syngamy, while haplosis, except in certain 

 algae and fungi, takes place at sporogenesis. The following sections 

 will deal chiefly with cytological features of particular interest in each 

 group, although this will entail some consideration of morphological 

 features described in textbooks of general botany. 



Gymnosperms. — The reproductive process in gymnosperms is carried 

 out in the cones. The staminate cone bears microsporangia in which 

 microspore quartets and eventually pollen grains are produced. The 

 male gametophyte in these grains has more cells than in angiosperms, 

 but the number of male gametes produced is practically always two. 

 The ovulate cone bears the ovules on the carpels which compose it, but 

 unlike angiosperm carpels these do not form an ovary enclosing the 

 ovules. This is the most fundamental distinction between the gynmo- 

 sperm cone and the angiosperm flower. 



In the ovule one megaspore of a quartet forms a coenocytic and then 

 multicellular gametophyte with archegonia, each containing one large 

 egg (Fig. 107). The female gametophyte in gymnosperms, which has 

 long been termed endosperm, thus develops far more extensively than 

 that of angiosperms before syngamy and, in addition to this, archegonia 

 are differentiated. In the Gnetales certain species resemble the angio- 

 sperms in differentiating their eggs in the coenocytic stage of the 

 gametophyte, the cellular stage following later, though with nothing 

 corresponding to a polar fusion. 



Syngamy in gj^mnosperms occurs in two main forms which differ 

 chiefly because of the character of the male gametes. In conifers the male 

 cells are nonmotile and are delivered to the egg through a pollen tube 

 which grows inward from the surface of the nucellus. In cycads (Figs. 

 107, 109) and the ginkgo tree the .young pollen tube grows not directly 

 toward the archegonium but into the tissue at the sides of the ovule, while 

 in the pollen grain end of the tube two very large motile spermatozoids 

 are differentiated. Disintegration of the nucellus allows this end of the 



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