74 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
however, if Pfeffer would say that in this case the cell, as a whole, 
had grown, but if he would, then his definition is a wide departure 
from ordinary usage. 
Vines offers a definition intermediate between that of Sachs and 
that of Pfeffer. “ By growth,” he says, “ we mean permanent 
change of form, accompanied usually by increase in bulk.” But 
then he goes on to say, “ Nor does an increase even of the organized 
structures of an organ, that is of the protoplasm and the cell wall, 
necessarily imply that it is growing. Thus, an increase of the cell 
wall may take place without any perceptible enlargement of the cell, 
as, for instance, when a cell wall thickens.” But since the thicken¬ 
ing of the cell wall is a “ permanent change of form,” it should be 
considered by the author a growth process, were not increase in size 
of the cell, after all, in the author’s mind, the most important 
criterion of its growth. Finally, Frank finds no other criterion for 
growth than an increase of volume (dependent, however, upon the 
increase of a particular substance). 
Thus, with these different plant physiologists we find the word 
growth bearing the ideas of increase of volume and of differentia¬ 
tion, then of differentiation alone, and, finally, of increase of volume 
alone. Returning now to the definition proposed above, we see that 
growth as mere increase of volume is to be distinguished from 
development, from differentiation, even from increase in mass, 
although the latter may often serve as a convenient measure of 
growth. 
In analyzing the processes of growth in organisms we must 
recognize at the outset that organisms are composed of living matter 
and formed substance, and that growth may therefore result from 
the increase in volume of either of these. The living; matter in turn 
is composed of two principal substances: the plasma and the 
chylema or cell sap ; so growth may be due to the increase of either 
of these substances, — may result either from assimilation, or more 
strictly from the excess of the constructive over the excretory 
processes of the plasma, or from the taking in of water. 
Other writers have analyzed the process of growth in very diverse 
ways. I cite a few of their conclusions. Huxley says, “ growth is 
the result of a process of molecular intussusception.” According to 
N. J. C. Muller, “ all phenomena of growth depend, in last analysis, 
upon this, that the molecule of the solid substance is introduced into 
the region of growth.” Frank understands by growth that increase 
