52 t>LAKT LIFE OK THE FAKM. 



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 



