CYTOLOGY OF REPRODUCTION IN ANGI06PERMS 145 



The antipodal cells usually degenerate and disappear during endosperm 

 development, but sometimes they enlarge and perform a nutritive func- 

 tion for a time. In maize the antipodals undergo division and eventuallj' 

 form in the kernel an oval mass of monoploid gametophytic tissue h'ing 

 next to the endosperm, which it closel}' resembles in cellular organization 

 and contained storage material. 



Alterations outside of the embryo sac may be briefly summarized here. 

 The tissue of the nucellus about the sac usually becomes less conspicuous 

 as the endosperm enlarges. In some plant families, however, it may 

 become stored with nutritive substances, in which case it is known as 

 pcrispcrm. The tissue of the ovule's integument (or integuments) 

 becomes transformed into the seed coat. The ovarian tissue enclosing 

 the ovule (or ovules) is variously modified into the fruit tissue, or pericarp. 

 This is represented by the pod of a bean, the hard covering of an acorn 

 or a maize kernel, the fleshy tissue of the tomato, and the fleshy tissue 

 (exocarp) and hard pit covering (endocarp) of the cherry. Accessory 

 fruits incorporate flower parts other than the ovary, as in the fleshy 

 tissue of the strawberry, which is developed from the receptacle, and in 

 the plume-like portion of the dandelion fruit, which is formed from the 

 calyx. In multiple fruits, such as the mulberry and pineapple, the tissues 

 of several flowers are combined. 



Finally, it should be pointed out that since the pericarp and seed coat 

 develop from tissues already present in the flower before pollination, a 

 given syngamic union does not alter the chromosomal constitution of 

 these parts but it does so affect the fruits and seeds of the next generation. 



Aberrations of the Reproductive Process. — In the angiosperms there 

 are encountered a number of modifications of the process of sexual 

 reproduction as described above. These occur only occasionally in 

 some plants, but in certain species one or more of them have become 

 habitual. On the whole they play a minor role in nature, but they should 

 at least be listed here because they do throw much light upon the problem 

 of lelationships within some genera and often afford an explanation of 

 luiexpected genetical behavior in the breeding plot. 



Several of these aberrations fall under the heading of apomixis, a 

 term applied to asexual reproductive processes substituted for the sexual 

 process. Apomixis maj^ be vegetative when buds or bulblets of various 

 kinds appear in the place of flowers. Of greater interest, however, 

 are the various types of apomixis involving the formation of seeds. 

 Examples of these are the following: (1) Reduced parthenogenesis, in 

 which an unfertilized egg develops with the gametic chromosome number. 

 This phenomenon has been observed in experimental plants belonging to a 

 number of genera, but it is not known to have become an established 

 habit anywhere among angiosperms in nature. (2) Unreduced partheno- 



