16 Principles of Plant Culture. 
cells of different sexes. It is not peculiar to the animal 
kingdom, but occurs in plants also, and except in rare cases, 
is neceseary to the production of seeds that are capable of 
germination (28). It is the only method of reproduction in 
the higher animals. Non-secual reproduction is independent 
of sex. It results from the direct separation of a part of 
the parent, which under favorable conditions develops into a 
complete individual. It occurs when plants multiply by means 
other than by seeds, as by non-sexual spores (53), bulbs (352), 
stolons (348), cuttings (358), etc., and it is a common method 
of reproduction in certain of the lower animals, as plant lice, 
(aphid). Reproduction does not usually take place until 
the period of most rapid growth has passed. 
18. Heredity and Variation. The offspring of a plant 
or animal tends to be like the parent or parents. But no 
two beings can be begotten and developed in exactly the 
same environment, and since environment always affects the 
individual more or less, it results that no two individuals can 
be precisely alike. Variation in the offspring may take 
place io any direction, as in the size or color of the flower, 
the tenderness or juiciness of the fruit, the prolificacy, the 
vigor (9), or the hardiness (10), etc. It follows, that in cul- 
ture, certain individual plants or animals are more desirable 
to the cultivator than others, because the individuals possess 
different qualities. 
19. The Principle of Selection. Since the offspring 
tends to resemble the parent or parents, by selecting the 
most desirable individuals for reproduction, we may gradu- 
ally improve plants or animals in the direction of greater 
usefulness. For example, by saving and planting seeds from 
the plants that produce the finest petunia or pansy blossoms, 
