STRENGTH AND ELASTICITY OF MECHANICAL CELLS 161 



below into the mesophyll, and thus form a sort of root for the firmer 

 attachment of the emergence. 



Typical sclerenchymatous cells, such as occur, for instance, in bark, 

 in the flesh of the Pear, or in the endocarp of a drupe, usually possess 

 greatly thickened walls, which are also con- 

 spicuously stratified ; frequently several 

 concentric compound strata can be clearly 

 distinguished. The secondary layers are 

 traversed by numerous pits, which are 

 often branched, and which are always 

 circular in cross-section (Fig. 5 1 a). The 

 walls are further commonly yellow or 

 reddish-brown in colour, and strongly 

 lignified. The protoplasts usually de- 

 generate ; the cell-cavities, which are 

 often greatly reduced, contain a watery 

 liquid or a reddish-brown granular 

 substance. 



B. STRENGTH AND ELASTICITY OF 

 MECHANICAL CELLS. 



In the preceding general discussion 

 reference has repeatedly been made to 

 the harmony which exists between the 

 histological structure of mechanical cells 

 and their functions ; in the present 

 section it will be demonstrated at some 

 length, that the chief physical properties 

 of the cell-walls of mechanical elements 

 are such that the cells in question are in 

 this respect also fully qualified to act as 

 the skeletal elements of the plant-body. 

 Schwendener 93 was the first to under- 

 take an experimental investigation of 

 the elasticity and tensile strength of 

 mechanical cells ; his experiments, which ] 

 deal principally with bast-fibres, are fully FlQ 53 



described in his classical treatise on the Aaha ^ ?Wularhairfrom the petiole 

 subject. Subsequently Ambronn made ^ m %g^5*^J*2& 

 a detailed study of the strength and 



elasticity of collenchyma. Data bearing on these questions have also 

 been obtained by von Weinzierl, F. Haberlandt (father of the author), 

 and others. 



