2O Plant Life and Evolution 



development is not so obvious. In the majority of 

 animals new individuals can arise only from the 

 specialized reproductive cells, and as a rule the 

 formation of a new individual is the result of the 

 fusion of two gametes, or sexual cells, the ovum 

 and spermatozoon, which are markedly different 

 from the somatic cells, i.e., the cells that compose 

 the various tissues of the body. In very many 

 plants there is a similar development of sexual cells, 

 and the process of fertilization is essentially the 

 same as in animals; but most plants also multiply 

 asexually, and not a few plants are known in which 

 this is the only method of propagation. Hence we 

 must remember that hereditary characteristics are 

 not transmitted by sexual cells alone. The plants 

 whose propagation is strictly asexual are not only 

 the lowest forms, like the bacteria, but also a good 

 many algse and fungi, some of them plants of large 

 size and complex structure. Among cultivated 

 plants are many which rarely or never produce 

 perfect seeds and are always propagated by division. 

 The banana, pineapple, breadfruit, and sugar cane 

 are examples of these, and the many varieties of 

 domesticated plants which have arisen in cultiva- 

 tion transmit their characters in a purely non- 

 sexual way. Bud variation is a not infrequent 

 phenomenon, and such variations are readily 

 perpetuated by cuttings or grafts. 



Asexual Reproduction in Plants. No cases are 

 known among the vascular plants, i.e., ferns and 



