728 RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 



with this demand the most different kinds of combinations of girders are seen 

 developed in it. Usually several, at least two, but often very many girders 

 are so combined that they traverse the axis in common, as shown in the dia- 

 grammatic cross-sections in figs. 177^'^'*. In this ease all the flanges are on 

 the periphery of the stem, and every pair — diametrically opposite one another — 

 must be regarded as belonging to the same girder. In many stems all the flanges 

 have a parallel course; in other cases they are bent in and out, and so connected 

 together as to form a lattice-work of the most complicated kind. In other cases 



Fig. 177.— Diagrammatic representation of various combined girders. 



1 A simple I (or double T) girder. 2 Two combined girders, arranged crosswise, s Three combined girders. * Six combined 

 girders ; the flanges are laterally in contact to form a cj'lindrical tube, s Four combined girders ; their Hanges are 

 formed of secondary girders. In Figs. •-* the web of the girders is indicated by dotted lines. 



all the flanges lying near the periphery of the stem are fused together (fig. 177*) 

 so as to form a cylindrical tube, in which case the web is not required and the 

 stem is either hollow inside, or is filled only with a loose pith. Sometimes each 

 separate flange is itself transformed into a girder, and in this way the flanges of 

 the chief girder become secondary girders, as represented in fig. 177*. There is 

 almost as great a variety in this matter as there is in the arrangement of the 

 strands of leaves, but since researches into the course and grouping of the strands 

 of mechanical tissue in stems are still not far enough advanced for us to be able 

 to place the various forms in well-arranged series, we must content ourselves with 

 sketching the most noticeable cases. 



