50 PLANT LIFE ON THE FARM. 



years ago the notion of movement taking place in plants, 

 other than that produced by the wind or other mechanical 

 agency, was, if not entirely ignored, so little considered 

 that the immobility of plants was contrasted with the 

 mobility of animals. We now know that even locomo- 

 tion is by no means an exclusive attribute of animals, 

 but for our present purposes we need only refer to 

 those movements more immediately connected with the 

 growth. 



Movement of Protoplasm. The protoplasm is a very 

 mobile substance, and the cell-membrane is very elastic, 

 while both, as has been shown, are permeable in various 

 degrees by water, the consequence of which is that under 

 favourable conditions the cells become turgid. As the 

 degree of turgescence varies according to circumstances, 

 tension being followed by flaccidity, and flaccidity overcome 

 by turgidity, it 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 con- 

 cerned must also take place. These movements are, of 

 course, more obvious when growth is irregular and un- 

 equal. Turgescence of the cells, as has been said, is an 

 essential condition of growth, and if this turgescence 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. 



