6 LECTURE I. 



1. it is absorptive, in that it is capable of taking up into 

 itself the substances which constitute its food : 



2. it is metabolic, in that it is capable of building up from 

 the relatively simple chemical molecules of its food the com- 

 plex chemical molecules of the organic substances present 

 in the cell ; and in that it is capable of decomposing the 

 complex molecules of these substances into others of simpler 

 composition : 



3. it is excretory, in that it gives off certain of the pro- 

 ducts of its destructive metabolism : 



4. it is reproductive, in that portions of it can become 

 separate from the remainder and lead an independent ex- 

 istence as distinct individuals. 



These are, however, by no means all the fundamental 

 properties with which the protoplasm of plants may be 

 endowed. The observation of the zoospores of Haematococcus 

 naturally suggests that their motility is due to some peculiar 

 property possessed by their protoplasm, and this is in fact 

 the case. When a zoospore is actively moving, its pointed 

 hyaline end bearing the cilia is directed forwards, and at the 

 same time the cell revolves round its long axis, so that it 

 advances in a screw-like manner. The movement is entirely 

 due to the cilia. So long as the organism is in motion, the 

 cilia are vibrating so rapidly that it is difficult to see them. 

 The lashing movement of the cilium is probably effected by 

 the alternate rapid shortening of each longitudinal half. To 

 this rapid shortening the term contraction has been applied, 

 and the body exhibiting it is said to be contractile. Since 

 the cilia are merely specialised portions of it, we may at- 

 tribute to the protoplasm of these cells the fundamental 

 property of contractility. It is obvious that the movements 

 of the zoospores involve the performance of a certain amount 

 of work ; the necessary energy is obtained from the destruc- 

 tive metabolism going on in the organism, which, as we have 

 seen, is necessarily accompanied by a conversion of potential 

 into kinetic energy. 



Further, it appears that the movements of the cilia origi- 

 nate independently of any external conditions which might 



