PROTOCOCCUS AND AMGEBA COMPARED. 183 



t 



duce its vital activities to a few fundamental pliysi(jl(jgical proper- 

 ties, namely, contractility, irritability, metaljijlism, grcjwtli and 

 reproduction, common to all animals. A little rejection will 

 show that the same properties are manifested also by Pmtn- 

 eoccus. Contraction and irritability are difficult to witness iu 

 the quiescent stage of Protococcus^ but obvious enougli in tlie 

 rarer motile forms. Metabolism, growth and reproduction, on 

 the other hand, are evident accompaniments of normal life, even 

 in the quiescent condition. And precisely as Protococcus diifers 

 from Amaiba in respect to contractility and irritability, of wliicli 

 it possesses relatively little, so plants in general differ in tliese 

 respects from animals in general. Animals are eminently con- 

 tractile and irritable, while plants are but feebly specialized in 

 these directions. On the other hand, as we have already seen 

 in comparing Pteris with Lumhricus (p. 154), and as we see 

 once more in comj)aring Protococcus with Amoeba^ in respect to 

 metabolism, the green plant is pre-eminently constructive, while 

 the animal is preeminently destructive, of organic matter. 



In their modes of nutrition, as stated above. Amoeba, 

 and Protococcus represent two physiological extremes. AVe 

 pass now to the study of Yeasts and Bacteria, which are plants 

 destiUite of chlorophyll and in a certain sense may be regarded 

 as occupying a middle ground between these extremes. 



Other Forms. There are innumerable species of uniceUular green 

 plants. A vast group of peculiar brownish forms covered with transparent 

 ^lass-like cells composed of siliceous material is known as the Diato- 

 macecE or diatoms. In these the chlorophyll is masked by a brown pig- 

 ment, but is nevertheless present. Another group is that known as the 

 Desmidice or desmids. These often have the individual cells peculiarly 

 constricted in the middle so that at first sight the two halves appear to be 

 two separate cells. More closely resembling Protococcus in many respects 

 are some members of the Cyanophycew or "blue-green algiv," among 

 which Ghro'ococcus and Glceocapsa differ from Protococcus chiefly, in the 

 former case, in having a blue-green instead of a yellow-green pigment, 

 and, in the latter, not only in this respect, but also in the fact that the 

 single cells are widely separated by transparent mucilage. 



