122 



OUTLINES OF PLANT LIFE. 



of thin-walled cells may, by the united tension of its com- 

 ponent cells, stretch those tissues which are not themselves 

 turgid. Such strains in the younger regions, particularly, 

 play an important role in maintaining the form of these 

 parts. But the tensions in the older parts are generally due 

 to the unequal growth of different tissues. (See ^ 218.) 



158. Mechanical rigidity. — The rigidity of the cell-wall 

 itself must be relied upon by all the larger plants. Certain 

 tissues are specialized by having their cell-walls greatly 

 thickened, and such tissue regions constitute a sort of frame- 

 work or skeleton, which is filled out by the more delicate 

 parts. These mechanical tissues are so distributed within 

 the body as to afford frequently the maximum resistance to 

 bending and breaking strains. 



In the accompanying diagrams the position of the mechanical tissues is 

 indicated in transverse sections of a number of different stems (fig. III). 

 It will be seen that they illustrate well-known mechanical principles in 



C D E 



Fig. Ill .—Diagrams showing the arrangement of mechanical tissues and vascular strands 

 in the cross-section of various stems. The mechanical tissue is gray ; the vascular 

 strands black, with white dots. A , linden (young); B, a mint ; C, a sedge ; Z), a 

 hamboo ; E, a grass. — After Kerner. 



their distribution. The hollow column {E) and the I-beam (A, B, C), 

 two of the most rigid mechanical constructions, are frequently imitated 

 in plants. 



