148 



BOTANY OF THE LIVING PLANT 



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

 of form is allowable in yielding to the strain 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. 



In the economi'^al 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 strains 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 strain. 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 



Fig. io6. 



Transverse section through a leaf of Cyperus, showing a 

 vascular strand with a strand of resistant sclerenchyma 

 above and below, constituting a girder. ( x 300.) F. O. B. 



I 



