4 MENDELISM CHAP. 



through a little hole in its coat and bores its way 

 down the pistil to reach an ovule in the ovary. 

 Complete fusion occurs, and the minute embryo of a 

 new plant immediately results. But for some time 

 it is incapable of leading a separate existence, and, 

 like the embryo mammal, it lives as a parasite upon 

 its parent. By the. parent it is provided with a 

 protective wrapping, the seed coat, and beneath this 

 the little embryo swells until it reaches a certain 

 size, when as a ripe seed it severs its connection with 

 the maternal organism. It is important to realise 

 that the seed of a plant is not a sexual cell but a 

 young individual which, except for the coat that it 

 wears, belongs entirely to the next generation. It is 

 with annual plants in some respects as with many 

 butterflies. During one summer they are initiated 

 by the union of two sexual cells and pass through 

 certain stages of larval development the butterfly 

 as a caterpillar, the plant as a parasite upon its 

 mother. As the summer draws to a close each 

 passes into a resting-stage against the winter cold 

 the butterfly as a pupa and the plant -as a seed, with 

 the difference that while the caterpillar provides its 

 own coat, that of the plant is provided by its mother. 

 With the advent of spring both butterfly and plant 

 emerge, become mature, and themselves ripen germ 

 cells which give rise to a new generation. 



Whatever the details of development one cardinal 

 fact is clear. Except for the relatively rare instances 

 of parthenogenesis a new individual, whether plant 

 or animal, arises as the joint product of two sexual 

 cells derived from individuals of different sexes. 

 Such sexual cells, whether ovules or ova, sperma- 



