GROWTH. 51 



Circumxmtation. Now, let us suppose the very frequent 

 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 per- 

 ceptible except by the use of delicate instruments, may some- 

 times 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 favourable to it. 



Movement of the Tip of the Boot. While the elon- 

 gation of the root near the tip takes place in the manner 

 described, the force of growth is not equal throughout 

 the whole region at the same time. Supposing the 

 fibril to be made up of cells piled up one upon another 

 in longitudinal rows, then the greatest energy of growth, 

 marked by the turgescence of the cells, occurs at one time 

 in one row, to shift at another time into the next row, and 

 so on in succession all round the root. The effect of this 

 greater turgescence and intensity of growth now in one 

 place, now at another is to move the tip of the root, not 

 in a circle, because growth is going on behind the tip as it 

 moves, but in an advancing spiral coil, so that the tip 

 is forced to enter the soil and to penetrate between its 

 particles, just as the point of a corkscrew is made by the 

 pressure of the hand to penetrate the cork, the pressure of 



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