114 THE VEGETATIVE FUNCTIONS OF PLANTS 



off by the growing cell. The immediate cause of all growth 

 is osmotic pressure. 



The great amount of force often exerted by young 

 growing organs, due to the osmotic pressure within their 

 cells, is strikingly illustrated in Fig. 72, showing the 

 rupturing of a concrete pavement by young fern leaves. 



Fig. 72. — The rupturing of concrete by the growth of young leaves of the 

 ostrich fern {Onoclea SlriUhiopteris Hoflfm.). (After Stone.) 



117. Experiment in Growth. — The relation between 

 osmotic pressure and growth may be demonstrated by a 

 very simple experiment, illustrated in Fig. 73. P is a 

 portion of the herbaceous stem of any convenient plant, 

 fastened securely at one end to an iron clamp (C), lying 

 at the bottom of a glass jar (J). The upper end of the 

 stem is attached by a small thread to the short arm 

 of an index (I), the opposite end of which may move up 

 and down over a graduated scale (S). If the jar is filled 

 with a solution of common salt water, water will pass out 

 of the plant tissue by exosmosis. This will reduce the 

 osmotic pressure {turgor) within, and the stem will shorten, 

 on account of the contraction of the elastic cell- walls, 



