174 ESSENTIALS OF BIOLOGY 



II ANIMAL AND PLANT REPRODUCTION 



When it reproduces an organism doubles itself and the char- 

 acters, form and potencies of the original organism appear in 

 each of the doublets. The " original " organism may be called 

 the " parent " and the doublets may be called the " daughter- 

 organisms." The daughters can be recognized as belonging to 

 the same " category " as did the parent (see Section 77). 



(This definition is of the nature of a " first approximation." 

 It is amplified in the following sections.) 



63. ON REPRODUCTION IN UNICELLULAR ORGANISMS 



A unicellular organism (a Protozoan in the cases of the animals, 

 or a Protophyte in the cases of the plants) is essentially a single 

 organic cell. Typically it reproduces by division, as described 

 in previous sections, and the division is probably one of mitosis 

 in most cases. But it is generally impracticable to observe, with 

 sufficient accuracy, the finer details in the very smallest organisms 

 (such as bacteria) and no doubt the process is greatly modified 

 with respect to the scheme in Section 62a. Thus the organism 

 doubles itself and ceasing to exist as one organism (the " parent ") 

 it simultaneously appears as two organisms (the '' offspring "). 

 The latter can be recognized as of the same kind as was the parent. 



The process is adequate for continued reproduction ad infinitum. 

 It is the only method by which Bacteria and Cyanophyaceae 

 reproduce. The giant Redwood trees are known to have lived 

 2,000 years and throughout that period they have grown — that 

 is, their cells have reproduced by simple mitotic division. Fibrous 

 connective tissue-cells are known to have existed in artificial 

 culture and to have divided thousands of times. So also with 

 some cultures of mice cancers kept in laboratories. 



63 fl. Senescence and Rejuvenation in the Reproduction 

 OF Unicellular Organisms. Some Protozoa can be kept under 

 laboratory conditions (that is, in small vessels or aquaria) and 

 continuously observed. All the individuals in such a culture, 

 or '* strain," are known to have resulted from the division of one 

 original organism. It has been observed that such a strain tends 

 " to die out." The rate of reproduction by division gradually 

 decreases until finally it ceases. Then the individuals of the 



