8 PRINCIPLES OF PLANT CULTURE 



essentially a process of division . As living cells multiply by 

 forming new cells, so living beings, which consist of cells, 

 multiply by the separation of a part of their own cells, 

 and this separated group of cells grows into a complete 

 organism like the parent. The higher plants multiply 

 by seeds (155), which are separated from the parent plant, 

 and each of which contains a young plant (53). The eggs 

 from which young birds are hatched contain cells filled 

 with living protoplasm, and the protoplasm of the living 

 young of mammals is separated from the parent before 

 birth. Prolificacy, the ability to reproduce abundantly, 

 varies greatly in both animals and plants. The quail 

 often produces twelve to fifteen young in a single season, 

 while two to four is an average number for the bluebird. 

 Prolificacy is also influenced to a lesser extent by variety. 



17. Reproduction either sexual or non-sexual. — 

 Sexual reproduction can take place, as a rule, only upon 

 the union of cells of different sexes. It is not peculiar 

 to the animal kingdom, but occurs in plants also and ex- 

 cept in rare cases is necessary to the production of seeds 

 that are capable of germination (28). It is the only 

 method of reproduction in the higher animals. Sexual 

 reproduction does not usually take place until the period 

 of most rapid growth has passed. Non-sexual reproduc- 

 tion 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 (52), bulbs (352), stolons (348), 

 cuttings (358), and the like, and it is a common method 

 of reproduction in certain of the lower animals, as 

 plant-lice (Aphids) . 



18. Heredity and variation. — The offspring of a plant 



