,Ro BOTANY OF THE LIVING PLANT 



rigidity is necessary, or the concrete would crack. But in the plant- 

 body, with its elastic cells and tenacious fibres, a considerable change 

 of form is allowable in yielding to the stress without permanent 

 injury following. The herbaceous plant thus has a distinct superiority 

 over any building of reinforced concrete, for the embedding medium 

 is itself elastic. The conditions are most nearly matched by the 

 covers of certain motor tyres, where resistance must be coupled with 

 elasticity, and fibre is embedded in the rubber. 



In the 'economical use of material the disposition of the specific 

 mechanical tissues is important, both on grounds of lightness of the 

 structure, and the physiological expense of the substance used. The 

 problem of obtaining the best mechanical effect with the least 

 expenditure varies with the requirements to be met. The girder 

 principle, which has been adopted by engineers as a means of 

 securing a high degree of mechanical efficiency with economy of 

 material, is frequently illustrated in the construction of plants. It 

 is even seen in plants of the Coal Period, such as Cordaites, which 

 lived ages before the origin of man. The common type is the double- 

 strap girder, which gives in transverse section the figure (i). If a 

 girder of such construction be fixed in the position indicated by the 

 figure, and loaded in the middle while it is supported at the ends, 

 there will be a tendency to curvature which will compress the upper 

 strap or flange, while the lower strap will suffer tension. The resistance 

 to these stresses will depend upon the two straps being held rigidly 

 in their relative positions by the connecting plate. The material will 

 be most economically used if it be concentrated in the form of the 

 upper and lower straps at the regions of greatest strain. The con- 

 necting plate may even be replaced in " latticed girders " by a system 

 of connecting ties, which follow the lines of greatest stress. The 

 wider the upper and lower straps are apart, consistent with their being 

 held rigidly in place, the better the result will be. The principle is 

 illustrated by the use of girders, or simple combinations of them, in 

 the construction of bridges, floors, and shop-fronts. More complicated 

 arrangements giving columnar construction are seen in lattice-signal- 

 posts, and large gasometer-frames. The latter offer close analogies 

 with certain types of stem-construction in plants. 



In plants girder-construction depends upon differences of mechanical 

 resistance of tissues. An illustration of a simple case is given in Fig. 1 13 

 from the leaf of Cyperus. On either side of the vascular strand there 

 is a band of thick-walled resistant woody sclerenchyma. Each is close 

 to the upper and lower surfaces respectively, indeed three cells of the 



