INBREEDING AND CROSSBREEDING IN SEED DEVELOPMENT 83 



those in the other great class of seed forming plants, the gymnosi)erms. A 

 secondary fertilization does not occur in the gymnosperms. The endosi)crm 

 is a haploid tissue derived from the megas])ore by continuous cell division. 

 The tissue is a part of the gametophyte rather than an integral structure 

 distinct from both gametophyte and sporophyte, as in the angiosperms. 



On the other hand, the endosperms in the two classes of seed plants have 

 an important common function, namely, nourishment of their respective 

 associated embryos. The genetic equipment with which the two kinds of 

 endosperms are furnished differs in a fundamental respect. That of the 

 gymnosperm is a sample half of the mother plant's inheritance, whereas the 

 angiosperm endosperm, being of biparental derivation, has two chances in- 

 stead of only one of receiving a physiologically effective genie complement. 

 Insofar as the two tissues are autonomous in their functional properties, the 

 angiosperm endosperm, therefore, is equipped to meet much more exacting 

 requirements than its counterpart in the gymnosperms. A summary review 

 of the differences in the gymnosperm and angiosperm ovules and seeds at 

 fertilization, and during the immediately subsequent period, shows the im- 

 portance of (or necessity for) a secondary fertilization in the flowering 

 plants in order to maintain continuity of the life cycle at this stage. 



The differences between the mature ovules of gymnosperms and angio- 

 sperms which appear to have a direct bearing on the present problem may be 

 summarized as follows: 



1. The seed coat in the gymnosperms approaches its mature size at the 

 fertilization stage. The angiosperm seed coat undergoes extensive growth sub- 

 sequent to fertilization. These facts are of interest in relation to the total 

 food requirements of the two respective classes of growing seeds and the 

 post-fertilization distribution of nutrients between the seed coat and the 

 enclosed tissues. 



2. The female gametophyte in the gymnosperms is an extensively de- 

 veloped multicellular (multinucleate, in some higher forms) structure. Its 

 counterpart in the angiosperms typically consists of only seven cells. The 

 potential disadvantage of the extreme reduction of the female gametophyte 

 in the flowering plants will be considered below. 



3. Generally speaking, the gymnosperm ovule is rich in food reserves, 

 whereas the angiosperm ovule is sparsely supplied. This means that in the 

 latter, the large volume of nutrients required for growth of the endosperm, 

 embryo, and seed coat must be moved in from other parts of the plant. In 

 the gymnosperms an extensive supply is directly at hand. 



4. So far as may be inferred from the published accounts, fertilization in 

 the gymnosperms initiates a new cycle of growth in the embryo only. Other 

 parts of the ovule do not appear to be stimulated. Double fertilization in the 

 angiosperms, in contrast, not only marks the inception of endosperm and 

 embryo formation, but also incites pronounced mitotic activity and en- 



