268 



EXPEEIMENTS WITH PLANTS 



that the construction is the best possible ; it is, as 

 you can easil}" see by experimenting with a strand 

 composed of several strings (not twisted together), 

 the construction that will stand the greatest pull. The 

 bracing roots (above ground) of the Corn, which have 

 to resist both thrust and pull, have strengthening 

 tissue both at the center and at the periphery. 



In the case of actively growing parts of plants we 

 have a different problem, since thick, woody cells, like 

 those of the wood and strengthening fibers, would not 

 be permissible ; we must have cells that are highly 

 elastic, so as to be easily stretched, thus permitting 

 v^ y^l Ib^^ the growth of the stem and yet 

 J?^w^ jL )L^ rigid enough to give stiffness. 

 The strengthening tissues of 

 ^( ,v A_>-%x/ iry this part of the stem (called the 

 -^J^^^^Ck) yfe=sl coUenchyma, Fig. 150) have 



these properties ; they are com- 

 posed of cells thickened at the 

 150. CoUenchyma. comcrs ouly ; they are able to 



grow T3y absorbing nutriment through the thin places 

 in their walls, and so keep pace with the growth of the 

 stem. The rigidity of the growing parts is helped by 

 the fact that the pith and internal tissues are com- 

 pressed by the outer ones, which grow more slowly, 

 and so set up strains (just as a spiral spring is more 

 rigid if placed in a cloth bag which is too small for it) . 

 Cut from the tip of a growing Elder stem (or stem of 



