PKOTOPLASTS IN COMMUNITY 251 



(p. 31), or a Chlamydomonas (p. 52), is a single proto- 

 plast fully equipped for existence in a particular environ- 

 ment. Multicellular plants are aggregates of proto- 

 plasts operating in community to mutual advantage, 

 and for the benefit of the individual plant of which they 

 are the living parts. The cell body, or protoplast, 

 secretes a cell wall which serves as a protective covering, 

 and also provides a degree of stability for the living unit. 

 Consequent upon the absorption of nutrient material, 

 and owing to its inherent vital activity, a cell grows. 

 When growth has proceeded to a maximum beyond 

 which distention of the protoplast would be dangerous, 

 the cell divides, and it is essential for us to appreciate 

 the fact that the growth of a multicellular plant involves 

 the continual increase of protoplasts. In the ordinary 

 course of thought we regard a flourishing Oak as a living 

 plant unit, but in point of fact what we see outwardly 

 is a wonderful habitation erected by milliards of micro- 

 scopic protoplasts. The Oak is due to the aggregation 

 and repeated multiplication of these vital units. It is 

 their home and their chemical laboratory. The wood 

 which gives it stability is composed of the hardened cell 

 walls of these industrious builders. In our previous 

 study of the one-cell plants we have observed that the 

 growth of the unit, which eventuates in division, leads 

 to an increase of individuals similar to the parent form, 

 and sufficient unto themselves. Moreover, we have seen 

 that propagation and vitality are stimulated by the 

 sexual act. In such plants the sexual act may take 

 place between any individuals, but in the multicellular 

 plant the cells multiplied by division remain in associa- 

 tion, instead of separating into independent units. They 



