8 Plant Life and Evolution 



approximate, since an essential condition of active 

 protoplasm is its great instability. As the result of 

 the activity within the protoplasm, new substances 

 are constantly being formed, some being con-/ 

 structive elements of the growing protoplasm, others 

 excretions which cannot be considered as truly a 

 part of the living protoplasm. Moreover, there may 

 be unassimilated food products from without. Nev- 

 ertheless, in a general way the essential chemical 

 composition may be determined, and we know that 

 the protoplasm of all organisms contains very much 

 the same chemical elements. The chemical elements 

 which are absolutely essential are comparatively 

 few, the most important being oxygen, hydrogen, 

 carbon, and nitrogen, which compose the greater 

 part of the protoplasm; but sulphur, phosphorus, 

 potassium, calcium, and iron are never absent from 

 normal green plants. Of course it must be assumed 

 that the proportion and arrangement of these are 

 different in different organisms. 



There is always present in active protoplasm a 

 large percentage of water which is necessary to put 

 the protoplasm in the semi-fluid, viscous condition 

 essential for its activity. Moreover, the water is 

 a solvent for most of the food elements needed for 

 the growth of the protoplasm. The water thus 

 serves a twofold purpose; first as a mechanical 

 agent putting the protoplasm in the labile condition 

 necessary for manifesting activity; and second, as 

 a means of transport of food in the form of solu- 



