DAVENPORT: THE ROLE OF WATER IN GROWTH. 83 
substance is growing substance — e. g ., the water does not reproduce 
itself. 
If then the increase of the body does not necessarily mean increase 
of the growing substance, there is no good reason for expressing the 
growth of any day in terms of the fraction of the initial weight 
which has been added during the day. If today all of the body 
substance is growing, an increase of 10 % will not mean so high a 
rate of growth as an increment of 10 % in a body half of which is 
water. If we assume for the moment, in our ignorance of what 
part of the body is growable, that the dry substance is all capable of 
reproducing itself (and that is an overestimate), we get a some¬ 
what different curve of percentage increments, as in Fig. 8b. The 
curve rises on the whole, but would have risen more rapidly if we 
had used only that fraction of dry substance which is growing. 
Here then we find no loss in the rate of growth of the growing 
substance, and in so far a lack of confirmation of Minot’s 
generalization. 
One reason for the lack of agreement between the curve of per¬ 
centage increments of the frog embryos and that of the guinea pig 
is due to the fact, that in our study of tadpoles we have been dealing 
with the beginnings of development, while in the guinea pigs, taken 
from birth on, we do not have the earliest stages. Doubtless a time 
will come in the life of the frog when it will not increase its weight 
a milligram a day; it will have ceased to grow. But are we justified 
in ascribing this cessation of growth to a gradual fading out of the 
growth force given by impregnation? We get some light upon 
this question when we go to plants and consider again the tip of the 
epicotyl. Here the growing tissue just behind the tip soon ceases 
to grow, while it undergoes histological differentiation. Is this ces¬ 
sation of growth due to the fading; out of a growth force — due to 
the remoteness of impregnation ? It is clear that this explanation is 
unsatisfactory, for the protoplasm at the very tip of the epicotyl 
continues to grow during, it may be, years. There is no evidence 
that it ever loses its capacity for growth. It may go on developing 
for centuries, as we see in trees. It may go on growing far beyond 
the ordinary life of a tree. Thus, as we all know, there is in 
Cambridge an elm “under which Washington took command of the 
American army, July 4, 1775,” and some cuttings have been made 
from this tree, which are growing quite as well as young trees that 
have presumably been derived from protoplasm more recently 
