CELLULAR STRUCTURE 39 



not motile, and the plant is furthermore 

 increased by budding and by outgrowths from 

 detached portions of thallus. The cells are 

 very regularly arrayed in the thallus, and the 

 conformation of the plant is clearly the result 

 of a co-ordination existing between the con- 

 stituent cells, and this is of a tolerably 

 advanced nature. Indiscriminate multiplica- 

 tions of the individual cells has been replaced 

 by a more ordered and ^regulated distribution 

 of the power of division amongst the cells 

 which together make up the Prasiola plant. 

 The relation of the frond to light is one of 

 the important factors in bringing this about. 

 But there is, besides this, and perhaps behind 

 it, a subtle intercommunicating influence 

 between the individual cells which together 

 constitute the colony, and this influence 

 determines the share that each is to take in 

 the building up of the organism as a whole. 

 Although we may find it impossible to identify 

 the exact nature of this influence in the 

 majority of instances, we know quite enough 

 to convince us that it is of a material nature. 

 We are well aware that the processes of growth, 

 and many other bodily functions besides, are 

 greatly affected by the presence of even 

 minute traces of certain substances, and the 

 physical approximation of the cells of an 

 organism renders it possible for substances 

 to diffuse from one to the other, and thus 

 to determine, in a plus or minus direction, 

 the rate of cell growth and multiplication. 



