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tions of the individual cells has been replaced 
by a more ordered and regulated distribution 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. 
