MECHANICAL STRENGTH AND RIGIDITY 51 



averages 11-12 kilogrammes (300-340 kgs. per sq. cm.), whereas the breaking 

 strain for a copper wire is about 6,000 kgs. per sq. cm. 1 



All the organs of a plant must at every period of their development 

 be able to resist all the stresses and strains they are called upon to bear. 

 In intercalary vegetative zones this is largely attained by the aid of 

 sheathing leaves, while in the case of foliage leaves with flexible petioles 

 and thin laminae the latter are set parallel to the direction of violent 

 winds by the torsion or bending of the stalk, and so lessen the 

 strain upon them. The impact of the water upon plants growing in 

 running streams is lessened in the same way, and in all such cases the 

 degree of flexibility is of considerable importance. 



Small plants have an advantage in many respects, but nevertheless 

 have relatively the same resistances to overcome as large plants. The 

 latter are, however, unable to exist in a foaming mountain torrent in which 

 small algae may cling to the stones, and marine algae are able to defy the 

 erosive action of the waves on rocky coasts. 



The existence of climbing plants shows that the stem need not always 

 have sufficient rigidity for its own support, and the same applies to many 

 aquatic plants which are buoyed up by the water in which they grow. In 

 all such plants, however, the osmotic pressure of the living turgid cells 

 enables them to resist considerable external pressures, and also the internal 

 tissue-strains due to growth. 



The cell-walls practically constitute the entire skeletal framework of 

 the plant, and the fact that this framework is typically composed of in- 

 numerable small chambers is of great importance, not only in the general 

 economy of the plant, but also in giving large ones the required rigidity. 

 Similarly, this minute subdivision enables succulent plants to acquire 

 sufficient rigidity in spite of the high percentage of water and small 

 amount of solid substance they contain. 



It is only in the case of trees, shrubs, and a few herbs, in which very 

 many of the cell-walls become hard and rigid, that the plant remains erect 

 after death. Most herbs and nearly all young or succulent organs droop 

 when the turgidity cannot be maintained owing to the loss of water, as well 

 as when it disappears on death. In such cases, the same physical causes 

 operate that make a bladder firm and rigid when air or water is forced into 

 it, and cause it to collapse and become flabby when its walls are no longer 

 stretched. Similarly, the tissue-strains aid in maintaining the rigidity of 

 growing organs, although such rigidity as is necessary in the trunks of 

 trees can only be attained by means of thick cell-walls. It would indeed 

 be a serious matter if the trunks of trees lost their rigidity and collapsed 

 whenever the water-supply was deficient. 



1 Cf. Ewart, Ann. du Jard. hot. de Buitenzorg, 1898, T. XV, pp. 194, 214. 



E 2 



