THE EVOLUTION OF PLANTS 91 



long delayed, and plants began to possess a bulk or 

 mass, instead of being filaments or one-layered plates. 

 This was a most important change, for it altered the 

 relations between the cells or protoplasts of the plant 

 and the surrounding water. Only the external cells in 

 such a mass were able to absorb water and the inner ones 

 had to depend on them for a supply. The external 

 cells, too, were those which ran the greatest risks from 

 changes of temperature in the water and from contact 

 with particles in it, or the many dangers which the 

 environment brings to the plant. These dangers and 

 difficulties must have increased as the plant itself grew 

 larger. The difficulties of the internal cells were 

 different, but no less serious. They were compelled to 

 draw upon the external ones for the renewal of the 

 water in their vacuoles, for the oxygen necessary for 

 breathing, for their food or its constituents, and for the 

 removal of any waste products they might produce. So 

 the two became gradually less and less like each other, or 

 more specialised. The gradual change of structure and 

 form of plants that followed can be traced in this way to 

 the need for adapting them to their conditions of life. 



We can follow with some probability the further 

 course of events. As the bulk increased the external 

 cells became still more devoted to protection and absorp- 

 tion ; the internal ones ceased to develop chlorophyll as 

 less light reached them, and the work of food construc- 

 tion which is the function of the chlorophyll was thus 

 thrown upon the outer layers. Among the internal 

 mass certain cells became concerned mainly in conduct- 

 ing the water to the rest, so supplying them with oxygen 

 and food. As the size became still greater the exterior 

 surface of the still symmetrical spheroidal or spherical 

 plant became inadequate to supply all that the inner 

 mass required; the spherical plant no longer had suffi- 

 cient surface in proportion to its bulk. This involved 



