52 PLANT LIFE ON THE FARM. 
come by turgidity, is of course obvious not only that 
changes of form must ensue from these differences in the 
degree of tension of the cells, but that movements of the 
parts concerned must also take place. These movements 
are, of course, more obvious when growth is irregular 
and unequal. Turgescence of the cells, as has been said, 
is an essential condition of growth, and if this turges-- 
cence take place on one side of a stem, or on one surface 
of a leaf only, a curve will be produced, the convexity of 
which will be along the line of greatest swelling and 
growth—the concavity on the opposite side where growth 
is less active, or altogether inoperative. The rapidly 
growing upper surface will be restrained as by a bridle 
by that part which is growing more slowly or not at all, 
and hence the curvature. 
Circumnutation.—Now, let us suppose the very fre- 
quent case where the greatest intensity of growth is now 
in one place, now in another, then, of course, we should 
have the curvatures first in one place, and then in 
another, and this is what happens in the case of growing 
shoots whose tips gradually revolve, forming circuits or 
ellipses of greater or less extent with greater or less 
rapidity, according to circumstances. This movement, 
which is not usually perceptible except by the use of 
delicate instruments, may sometimes be watched by the 
naked eye, even in the case of such apparently stiff parts 
as the leading shoots of Firs. Among other objects 
gained by this movement of ‘‘revolving nutation,” or as 
Darwin called it, ‘‘ circumnutation,” is the exposure of 
each leaf in turn to the conditions of light most favor- 
able to it. 
Movement of the Tip of the Root.—While the elon- 
gation of the root near the tip takes place in the manner 
described, the force of growth is not equal throughout 
