CHAPTER X 

 CYTOLOGY OF REPRODUCTION IN ANGIOSPERMS 



The life cycles of vascular plants, brj^ophytes, and many thallophytes 

 are complicated by the presence of an alternation of generations. 3y this 

 it is meant that the cycle comprises two phases of vegetative development : 

 one of these, the sporophyte, develops from the zygote and produces 

 spores; the other, known as the gametophyte, develops from the spore and 

 produces gametes. Since the chromosome number is reduced at sporo- 

 genesis and doubled at syngamj^, the sporoph\i:e normally has twice as 

 many chromosomes as the gametophyte. 



The two "generations " differ greatly in relative degree of development 

 in the various plant groups. In most bryophytes the gametophyte is 

 the more conspicuous: a moss plant, for example, is a gametophyte, the 

 sporophyte being small and short-lived. In ferns this relationship is 

 reversed, the gametophyte being so small that it commonly escapes notice. 

 In flowering plants the gametophyte is still more obscure and must be 

 studied \wiih the microscope. 



The cycles are complicated further by variations in the type of sex 

 differentiation. In mosses and ferns the sperms and eggs may be pro- 

 duced by the same gametophyte (monoecism, homothallism) , or they may 

 occur on different ones (dioecism, heterothallism) . In seed plants, male 

 and female gametophytes are always distinct and arise, respectively, 

 from microspores and megaspores. These spores may be produced by 

 the same sporophj^te, in which case the plant is homophytic (monoecious 

 or hermaphroditic). In other species they are produced by different 

 sporophytes; such species are heterophytic (dioecious). The cytological 

 basis of these sexual conditions will be discussed in Chap. XII. 



The present chapter will be devoted entirel}^ to the angiosperms, for 

 these are the plants that have long been most prominent in cytological, 

 cytogenetical, and cytotaxonomic researches. 



The Flower. — Under the appropriate physiological conditions the 

 angiosperm sporophyte produces flowers. The typical flower is a group 

 of parts — pistil, stamens, petals, and sepals — of which the first two kinds 

 are directly concerned in sexual reproduction. In the anthers of the 

 stamens, microspores, later becoming pollen, are formed. In the ovarian 

 portion of the pistil, ovules with megaspores are produced, and it is there 

 that the subsequent stages of female gametophyte development, syngamy, 

 embryogeny, and seed formation are carried out. 



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