II] THE BEACH AND ROCKS 19 



cells of a filament like that of Ulothrix be divided 

 longitudinally as well as transversely, the transition 

 to such forms as those just described would be easy: 

 and judging from the fact that all these Algal types 

 begin life as simple filaments, this conclusion seems 

 to be a highly probable one. AVhen it is noted 

 further that the minute cells composing these Algae 

 are essentially like those of Ulothrix, the mode of 

 their origin can hardly be held to be any longer in 

 doubt. 



In plants at large a quiescent period of self- 

 nutrition precedes propagation, and these Algae are 

 no exception. It matters not whether it be a simple 

 filamentous plant like Ulothrix, or a large expansion 

 such as Ulva, during this first period it is capable of 

 growth accompanied by cell-division to which there 

 is no ostensible limit. The relatively large body thus 

 produced, consisting of thousands of cells in such 

 a plant as Ulva, is apparently inert, for the living- 

 protoplasts are enclosed and fettered each by its 

 restricting cell-wall (Fig. 1, p. 17). The vegetative 

 period is thus one of bodily quiescence, however great 

 the physiological activity may be. But when in any 

 matured and well-nourished plant the circumstances 

 are favourable for propagation, a change may occur 

 in individual cells, which leads to a period of very 

 active movement. Their protoplasts, after sub- 

 division, escape from the restricting cell-walls into 



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