76 HOLISM AND EVOLUTION chap. 



evidence to give. Differentiation in cells must have com- 

 menced as soon as the daughter cells began to adhere to 

 the parent cell and multicellular organisms were formed. 

 In the unicellular Pleurococcus, which is about the simplest 

 plant form known, noticed as the green slime on the damp 

 bark of trees or wooden posts, we see the beginnings of 

 this process of cell aggregation, as daughter cells adhere to 

 the parent cell until several divisions have taken place and 

 only then separate into individual cells. The Pleurococcus 

 cell is globular, but during this attachment the cells are 

 flattened at the surfaces of contact. In the multicellular 

 organisms a layer of cuticle covers the outer cell walls in 

 contact with the air and retards the loss of water. Step by 

 step other differentiations appear, and the plant body 

 becomes more complex as we advance from alga to fern, 

 and from fern to the higher seed-bearing plants. The 

 differentiations into various organs, such as the root, stem, 

 leaf and reproductive organs, are simply means towards the 

 division of physiological labour. Thus in cells away from 

 the light photo-synthesis is impossible, and they become 

 dependent on the outer green cells; similarly the roots under- 

 ground become dependent for starch on the green cells and 

 in return absorb dissolved salts for supply to the rest of the 

 plant. The water requirements render necessary the fibro- 

 vascular cell system, while the reproductive functions become 

 confined to special organs. All this differentiation means 

 more organisation and a more elaborate structure of the 

 plant. In addition to this division of labour the struggle 

 for existence tests the structure in other directions, and 

 means more modification in response to the stress of the 

 struggle and the stimulus of the environment generally. 

 As a result the plant structure comes to be elaborated and 

 adapted to the inner and external demands upon it, and to 

 assume the forms which are known to us. 



Besides this general differentiation and organisation, 

 there are special causes which have brought about the 

 divergence of plant and animal forms. A consideration of 

 these matters falls outside the scope of our task. Generally 

 it may be said that plant-life has been determined and 



