LECTURE X. 



THE types which we have examined so far, Chlamydomonas, 

 Saccharomyces and Spirogyra, are unicellular plants. That is, 

 each cell is a complete plant and discharges all the functions of 

 life. It assimilates, respires, frees itself from waste-products, 

 grows and finally reproduces itself. In Chlamydomonas the cells 

 break apart as soon as they are formed and pursue a separate 

 existence so that there can be no question that each separate 

 individual consists of a single cell. The same appears true of 

 Saccharomyces, in which as soon as the budded cells have attained 

 a mature size or even earlier they are so easily separated from 

 one another that under normal conditions each becomes inde- 

 pendent and carries on a separate existence. But it was also 

 seen that under special conditions for example, in the gelatine 

 culture the separate cells remained together and formed a 

 branching system which might remind one of the higher plants. 

 However, this coherence is purely accidental and the individual 

 cells, notwithstanding their proximity to one another, are each 

 complete individuals and discharge all the functions of an in- 

 dividual. The accidental coherence of the individuals of 

 Saccharomyces naturally leads us to the case of Spirogyra. Here 

 coherence is more than accidental and is the general rule. But 

 notwithstanding this fact, the separate cells of Spirogyra are rightly 

 regarded as individuals, and the filament as a colony. Each 

 carries on its own life -processes independently of the rest. In 

 the manner of carrying on these processes they are not influenced 

 by their neighbours, and the protoplasm of each cell is completely 

 isolated from the rest by its cell-wall. Only in their sexual re- 

 production do they show any differentiation, and that differentia- 

 tion, such as it is, is a differentiation of individuals which in this 

 case are unicellular. Such unicellular individuals, which not only 

 discharge all the vegetative functions but also give rise by division 

 to new individuals, are, as has been pointed out, putting the chance 

 of accident on one side, immortal. 



In this lecture we come to the study of an organism which 



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