Variation and Heredity 477 



It finds direct practical application in all the practices 

 of plant and animal breeding. 



289. Nonsexual reproduction and heredity.- -In non- 

 sexual reproduction a part of an individual reproduces a 

 new individual, and the latter commonly resembles the 

 former as closely as environment or the conditions of its 

 growth will permit. In this case we may scarcely speak 

 of heredity or transmission in the usual sense ; yet it is 

 important to recall that whatever the nature of the part 

 used for such vegetative multiplication, it retains the racial 

 or specific characteristics of the plant from which derived. 

 The propagative part or scion usually includes one or 

 more buds. Reduced to the lowest terms conceivable, it 

 might be a single vegetative cell. In at least tw r o cases 

 among flowering plants an epidermal cell may develop a 

 bud, and this bud reproduces the plant. In any case it is 

 remarkable that a single cell, or even a group of meriste- 

 matic cells, should be able to reproduce so completely 

 all the qualities of a complex adult. Bud variations may 

 occur in plants propagated by nonsexual means, yet most 

 clonal varieties are said to be fairly constant. 



290. Sexual reproduction and heredity. - In sexual 

 reproduction, individuals contribute characteristics through 

 the two gametes or uniting cells, and through this union 

 two lines of ancestry are united in one organism. As 

 already noted, in the angiosperms these gametes are a 

 nucleus, with little or no accompanying cytoplasm, from 

 the pollen tube and an egg cell in the ovule ; these micro- 

 scopic, protoplasmic units must carry all the characteris- 

 tics entering into the organism. This organism will not 

 commonly resemble in absolute detail either parent, nor 



