.SECT. II PHYSIOLOGY 267 



times, and that of the rye haiihii by five hundred times, the diameter of tlie base. 

 In addition, moreover, to the great disproportion between tlie height and diameter 

 of plants, they often support a heavy weight at the summit ; the Rye straw must 

 sustain the burden of its ear of grain, the slender Palm the heavy and wind-swayed 

 leaves (which in species of Ila2)hia have a length of 15 m. and a corresponding 

 breadth), while at times the 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 

 liossess instead a wonderful degree of ELASTicrrr. 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 them- 

 selves, but perfectly adapted to their needs. The firm but at the same time elastic 

 nuiterial 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 nuinufacture of thread and cordage. The sclerenchy- 

 matous elements of these tissues are the main mechanical elements of the plant ; 

 the collenchyma also plays an important part. 



Selerenehyma. — The firm thick walls of sclerenchymatous cells and 

 fibres are not infrequently further hardened by deposits of mineral 

 substances. The resistance which these forms of tissue offer Avhen the 

 attempt is made to cut, tear, or break them affords sufficient evidence 

 of their hardness, tenacity, and rigidity. Moreover, Schwendkner 

 has been able to determine their mechanical value by means of exact 

 physical experiments and investigations. According to such estimates, 

 the sustaining strength of sclerenchymatous fibres is, within the limits 

 of their elasticity, in general equal to the best wrought iron or 

 hammered steel, while at the same time their ductility is ten or fifteen 

 times as great as that of iron. It is true that soon after exceeding its 

 limit of elasticity the stereome of the plant becomes ruptured, while 

 the modulus of rigidity for iron is not reached until the load is in- 

 creased threefold. It is, however, of value for the needs of the 

 plant that its limit of elasticity extends almost to the limit of its 

 rigidity. 



Collenchyma. — The selerenehyma corresponds to the bony skeleton 

 of the animal body. Its elements are no longer in a condition Avhich 

 allows of growth, and it cannot be employed in parts of the plant 

 which are still actively elongating. Where such parts of the plant 

 require special strengthening in addition to that given by the tensions 

 of cells and tissues, this is obtained by means of collenchyma (pp. 75, 

 127). This tissue in spite of the large amount of water contained in it, 

 possesses considerable resistance against tearing. At the same time 

 it can yield to the growth of surrounding tissues since it is itself 

 capable of active growth. The collenchyma may be said to represent 

 the cartilaginous tissue of the plant. 



Just as the mechanical tissues of the internal framework of plants 

 exhibit the physical properties most essential for their purpose, their 

 arrangement, as SchwenDjSNER showed, will also be found equally 



