PROTOCOCCUS AND AMCEBA COMPARED. 183 



duce its vital activities to a few fundamental physiological proper- 

 ties, namely, contractility, irritability, metabolism, growth and 

 reproduction, common to all animals. A little reflection will, 

 show that the same properties are manifested also by Proto- 

 coccus. Contraction and irritability are difficult to witness in 

 the quiescent stage of Protococcus, but obvious enough in the 

 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 differs 

 from Amoeba in respect to contractility and irritability, of which 

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

 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 Lumbricus (p. 154), and as we see 

 once more in comparing 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. We 

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

 destitute 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 unicellular green 

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

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

 maceoe 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 Cyanophycece or "blue-green algae," among 

 which Chroococcus 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. 



