198 BOTANY: PRINCIPLES AND PROBLEMS 



gamete or egg. With the nucleus of this egg cell fuses one of the 

 male gametes which has come down the pollen tube (Fig. 110, C). 

 This union produces the fertilized egg, and from this single cell 

 develops the entire embryo of the seed and thus the young plant 

 which grows therefrom. This fertilized egg, in which are com- 

 bined the protoplasm of the two parents, is the sole direct link 

 between parents and offspring; and only across this exceedingly 

 narrow bridge are characteristics transmitted by inheritance 

 from one generation to the next. 



The fertilization of the egg by a male cell is not the only cell 

 union which takes place at this time, for the other male cell 

 fuses with the endosperm nucleus (Fig. 110, C), and from the 

 cell thus formed arises the endosperm or food-storage tissue 

 of the seed. 



Fertilization effected by gametes from the same plant is known 

 as self-fertilization; that by gametes from different plants as 

 cross-fertilization . 



Seed Development. — After fertilization has been effected, 

 the petals and stamens drop off and the ovule gradually develops 

 into the seed (Fig. 110, D). Various changes accompany this 

 process. The whole structure grows markedly in size and the 

 integuments increase in thickness, become hard and woody, 

 and close over the micropyle. In many seeds a considerable 

 mass of endosperm or food-storage tissue is developed, but in 

 others this is much less abundant. Within the endosperm is the 

 embryo or young plant, which has developed from the fertilized 

 egg. In dicotyledonous plants (p. 360) the embryo is differen- 

 tiated into three main portions; the hypocotyl or primitive stem 

 and root, its tip directed toward the micropyle; the two seed- 

 leaves or cotyledons, attached to the upper end of the hypo- 

 cotyl, and the plumule or bud, inserted between the cotyledons 

 (Fig. 112). The cotyledons may be very thick and serve entirely 

 for food storage, as in the pea; they may be thin and leaf-life, 

 serving as foliage leaves from the beginning, as in the morning 

 glory, or they may combine both functions, as in the squash. 

 In monocotyledonous plants (p. 364) endosperm is always well 

 developed and the comparatively small embryo consists of a flat 

 disc, the scutellum (which probably represents a single cotyledon) 

 to the face of which are attached an upward-pointing, sheathed 

 plumule or bud and a downward-pointing minature root or 

 radicle (Fig. 113). The scutellum serves to absorb food from 



