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 Slruthiopteris Hoffm.)- (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, 
