178 BOTANY 



I. The Stability of the Plant Body 



One of the most important and essential physical attributes of a 

 plant is its rigidity. Without that quality plants could retain no 

 enduring form. Plants which reach a considerable height and by 

 branching expand freely in the air are specially dependent on the 

 rigidity of their bodies so as to remain fixed in the position they 

 have attained. The capacity to return, by their own independent 

 movement, to favourable positions from which they may have been 

 forcibly displaced by external influences, is, in trees and shrubs, and 

 also in the more rigid herbs, restricted to the extreme tips of the 

 growing branches. 



How great are the demands made upon the stability of plants will 

 be at once apparent from a consideration of a Rye haulm ; although 

 it is composed of hundreds of thousands of small chambers or cells, 

 and has a height of 1500 mm., it is at its base scarcely 3 mm. in 

 diameter. The thin stems of reeds reach a height of 3000 mm. with 

 a base of only 15 mm. diameter. The height of the reed exceeds by 

 two hundred times, and that of the rye haulm by five hundred times, 

 the diameter of the base. In addition, moreover, to the great 

 disproportion between the height and diameter of plants, they 

 often support a heavy weight at the summit ; the Rye straw 

 must sustain the burden of its ears of grain, the slender Palm the 

 heavy and wind-swayed leaves (which in Lodoicea seycJiellarum have a 

 length of 7 m. and a breadth of 3-4 m.), while at times the consider- 

 able weight of the bunches of fruit has also to be considered. 



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

 required, and they possess instead a wonderful degree of ELASTK ITY. 

 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 used in the manufacture of 

 thread and cordage. 



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 receptable such as a rubber tube, the walls of the tube become 



