266 EXPERIMENTS WITH PLANTS 



lengthens and the lower shortens, while midway be- 

 tween them, along the line AB, there is no tendency 

 either to lengthen or to shorten. We do not, therefore, 

 need so much material along this line, and may trans- 

 fer a large part of it to the upper and lower surfaces, 



where the greatest 

 strain comes. By 

 so doing we make 

 ^ ' a girder (Fig. 147) 



147. Diagram of a girder. which COntaiDS the 



same amount of material as the beam but will bear a 

 much greater load. On the same principle, a hollow 

 cylinder will bear a greater load than a solid one con- 

 taining the same amount of material. Do you find the 

 principle of the girder and hollow cylinder employed in 

 the construction of the stem? On examining the cross- 

 section of an herbaceous stem under the microscope, 

 we find the thick -walled cells partly in the wood and 

 partly in the strengthening fibers (Fig. 131, sir) which 

 may surround the bundle, as in the Corn 1 (Fig. 134, sir] , 

 or may lie external to it. In all cases the strands of 

 strengthening fibers are connected with each other or 

 with the strands of wood by intervening tissue so that 

 they act as the flanges of girders (see Fig. 148), or 

 else they form hollow cylinders (as in Fig. 131). It 



* !It will be noticed in the figure that, at the sides of the bast and there- 

 abouts, the strengthening fibers form only a very narrow layer, so as not to 

 prevent the passage of materials from the bast and wood to the pith at 

 this point. 



