THE CELL 223 



bodies or plastids. Certain plastids present in the 

 majority of plants are of particular importance as con- 

 taining the green substance chlorophyll, which plays 

 an essential part in the fixation of carbon from the 

 atmosphere. 



Amongst unicellulai organisms the creatures 

 already mentioned as being made up of a single cell 

 only those which contain chlorophyll and are pro- 

 vided with a firm cell wall, built up of a material 

 known as cellulose, are usually regarded as simple 

 plants; whilst those in which chlorophyll and a cell 

 wall are absent are looked upon as simple types of 

 animals. Similarly slight differences distinguish the 

 cells which build up the fabric of the higher plants 

 from those of which the bodies of the more complicated 

 animals are composed, so that in almost all essential 

 points an account of the behaviour of the cells of the 

 members of one kingdom will apply equally well to 

 those of the other. After a few further preliminary 

 remarks we shall, therefore, for the sake of simplicity, 

 speak of a generalized type of cell, the behaviour of 

 which, except in points of detail, will resemble that of 

 the actual cells of plants or animals indifferently. But 

 in order to convey a more definite idea of an uni- 

 cellular animal to those who are unfamiliar with the 

 now flourishing science of Protozoology, we may refer 

 briefly to the well-known form Amceba, which will 

 serve as an excellent type of an animal consisting of a 

 single free-living cell. 



This little creature consists of a mass of protoplasm 

 enclosing a nucleus which is more or less centrally 



