CHAPTER TEN 



FERTILIZATION 



1. REPRODUCTION comes about through division. In Reproduc- 

 the simplest forms of life trie cell is the individual, and it division* 

 divides into two equal parts, each of which feeds and 



grows to the size of the original cell. In the many- 

 celled animals and plants, that is, all the higher 

 forms, cell division does not usually give rise to new 

 individuals. As in the first case, new cells are pro- 

 duced, but they are retained as part of the body of the 

 creature. Sometimes, however, a mass of these cells 

 is set apart, forming a bud or similar structure, which 

 may break away and become a separate individual. 

 Thus the Hydra, a small ccelenterate animal found in 

 ponds, produces buds which develop into what seem to 

 be little hydras parasitic on the mother. These pres- 

 ently break away, and become new individuals. Many 

 plants reproduce by runners or tubers ; in some kinds 

 of sunflowers the original plant dies, while its under- 

 ground branches produce new plants the following year. 

 In Arizona certain branching cacti rarely produce seed, 

 but their branches break off and take root where they 

 fall, thus producing new plants. 



2. In all the cases just cited there is no fertilization, Conjugation 

 MI the biological sense. It is found, however, that even J orms of 

 among the one-celled animals (Protozoa) conjugation fre- Ufe 

 quently takes place, though apparently not essential. 



This conjugation consists in the union of two individ- 

 uals ; in Paramecium these come together, exchange 

 portions of their protoplasm, and then separate. Each 

 may be said to have fertilized the other, by giving it a 

 portion of its substance ; neither has lost in bulk, since 

 the exchange is equal. In other cases, as for instance 



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