PHYSIOLOGY 165 



length and 12 m. in breadth. In the case of free-growing plants, 

 which attain the height of high buildings, e.g. Eucalyptus and Seijuoia, 

 the proportions noticed in a single grass-haulm no longer obtain. 



In plants, however, the rigid immobility of a building is not 

 required, and they possess instead a wonderful degree of elasticity. 

 The rye straw bends before the wind, but only to return to its 

 original position when the force of the wind has been expended. 

 The mechanical equipment of plant bodies is peculiar to themselves, 

 but perfectly adapted to their needs. The firm but at the same 

 time elastic material which plants produce, is put to the most varied 

 uses by mankind ; the wood forms an easily worked yet sufficiently 

 durable building material, and the bast fibres are employed for a 

 variety of economic purposes. 



In young stems and plants, in which the stiff but elastic wood 

 and sclerenchymatous fibres are not developed, the necessary rigidity 

 cannot be attained in the same way as in the older and woody stems. 

 But although the principal component of such young stems is water 

 (often 90 per cent or more), they maintain a remarkable degree of 

 rigidity and elasticity through the elastic tension of their extremely 

 thin and delicate cell walls. 



Turgidity. — When air or water is forced, under pressure, into an 

 elastic receptacle such as a rubber tube, the walls of the tube be- 

 come stretched and the tube longer and thicker. By this process 

 the tube becomes just so much stiffer and firmer the greater the 

 internal pressure and the more elastic and thinner its wall. By the 

 similar tension of their elastic cell walls arising from internal pressure, 

 the rigidity and elasticity of thin-walled plant cells, and organ's com- 

 posed of them, are maintained. The cellulose walls of parenchymatous 

 cells are, in spite of their delicate structure, exceedingly firm and, 

 at the same time, elastic ; when distended, therefore, by a strong 

 internal pressure they exhibit physical properties similar to those of 

 a rubber tube. In order to understand how such an internal pressure, 

 actually existing within a cell, can arise, it is necessary to take 

 into consideration the physical phenomenon of osmosis, first in- 

 vestigated by the botanist Dutrochet, and later more particularly 

 studied by Pfeffer and De Vries. Disregarding the recent and as 

 yet merely theoretical views, according to which osmotic pressure, like 

 that of steam, is supposed to be derived from the impact of motile, 

 isolated molecules or ions against the walls, it will be assumed that 

 osmosis is due simply to the mutual attraction of small particles of 

 solid matter and their solvents. It depends also on the molecular 

 attraction which converts solid bodies into solutions, and which so 

 operates that the dissolved substances become uniformly distributed 

 throughout the solution. 



When two solutions of unequal concentration are separated by a 

 membrane which is equally permeable to both, an attraction and 



