DIV. I 



MORPHOLOGY 61 





of lamellae of carbohydrate material, which is often lignified. The 



sclerenchymatous cells or stone cells (Fig. 30) are more or less 



isodiametric and polyhedral and have round, branched, or unbranched 



pits. The sclerenchymatous fibres (Fig. 61), on the other hand, are 



narrow, elongated, spindle-shaped cells with pointed ends, polygonal 



in transverse section (Fig. 62). They have obliquely-placed, narrow, 



elliptical pits. In their development sliding growth 



frequently occurs and they only mature in fully- - 



grown parts of the plant. These elements may 



occur singly, but usually, especially in the case of 



the fibres, they are closely associated in strands, 



bands, rings, or sheaths, variously arranged so as to 



ensure the requisite., rigidity of the organ against 



bending, tension, or pressure while employing the 



least mechanical tissue. 



The firm thick walls of sclerenchyraatous cells and fibres 

 are not infrequently further hardened by deposits of mineral 

 substances. The resistance which these forms of tissue offer 

 when the attempt is made to cut, tear, or break them affords 

 sufficient evidence of their hardness, tenacity, and rigidity. 



Sclerenchymatous fibres have always a length which for a 

 plant cell is considerable, on the average 1-2 mm. In some 

 plants they are much longer, e.g. 20-40 mm. in Flax, to 77 

 mm. in the Stinging Nettle, and in Boehmeria even 220 

 mm. Such long fibres are of economic importance in the 

 manufacture of textile fabrics. The long pointed ends render 

 the connection of the fibres more intimate than is the case for 

 the cells of other tissues. 



SCHWENDENER has been able to determine their mechanical 

 value by means of exact physical experiments and investi- 

 gations. 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 extensibility FJG gl. A scleren- 

 is ten or fifteen times as great as that of iron. It is true chymatous fibre, (x 

 that soon after exceeding its limit of elasticity the stereome about 100. After 

 of the plant becomes ruptured, while the limit of rigidity STRASBURGER.) 

 for iron is not reached until the load is increased 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. 



(b) Collenehyma. The sclerenchyma corresponds to the bony 

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

 which 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. 



The collenchymatous cells may be isodiametric but are usually 

 elongated ; they have transverse end walls (Fig. 64) or are pointed. 



