166 



MECHANICAL SYSTEM 



The strength of a girder depends, ceteris paribus, upon the strength 

 of its fiances. It also increases as the distance between the two 

 flanges becomes greater, because the tension due to the load varies 

 inversely as the distance between the flanges. Since the web has to 

 bear only a small proportion of the total strain, it may be considerably 

 lighter in construction than the flanges which it links together. In 

 the girders of bridges the web usually takes the form of a lattice- 

 work or honey-comb structure; if a girder is composed of more than 



one kind of substance, the inferior material is 

 used in the construction of the web. Similarly 

 in the plant the flanges of a girder are always 

 composed of mechanical cells, whereas the web 

 may consist of vascular tissue or of paren- 

 chyma. 



A simple I-girder resists bending only in 

 a single plane. In order to obtain a structure 

 which is inflexible in several planes i.e. in 

 several directions at right angles to the long 

 axis of the organ several I-girders must be 

 combined in such a way that they have a 

 common neutral axis. 96a The accompanying 

 diagram (Fig. 55 b) shows such a combina- 

 tion comprising three I-girders, a a, bb, and cc 

 being the respective pairs of flanges. The 

 broken lines represent imaginary webs, which 

 are superfluous here because adjoining flanges 

 are firmly joined together by means of tangen- 

 tial connections. In this way the two flanges 

 pertaining to each girder are connected just as securely as if the web 

 were actually present. Compound girders of this kind are exceedingly 

 widespread in vegetable organs. By supposing a number of girders 

 to become so closely crowded in a ring, that their flanges undergo 

 lateral fusion, one arrives at the conception of a hollow cylinder or tube, 

 likewise a very prevalent type of mechanical construction among plants. 

 Each of the numerous flanges of a radially inflexible compound 

 girder may be exposed alternately to compression and tension. In 

 order that the individual flanges may not warp or buckle, when 

 compressed, before the elastic limit of the tissue is reached, each is 

 also constructed so as to resist bending, its cross-section resembling 

 that of an entire girder, with the outline of an I, a cross or ring, or 

 other appropriate figure. Such an adaptive modification, whereby 

 the individual flanges of the main girders are themselves converted 

 into secondary girders, is also frequently found in the plant-body. 



A. Cross-section of an I-girder; 

 (/, upper flange ; g', lower flange. 

 B. Cross-section of a compound 

 girder, comprising three I-girders, 

 na', bb', cc' , with their common 

 neutral axis at o. 



