RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 727 



architectural elements in a still growing portion of the stem; they would either 

 prevent the lengthening of the other tissues, or would be ruptured by the force 

 of the elongating cells, and in both instances would be injurious. The collenchy- 

 matous cells, on the contrary, are able to continue developing, they can elongate 

 and grow with the other tissues, and may be compared with the scaffolding of a 

 several-storied building, which is constantly being raised as the work progros.ses. 

 The collenchyma, of course, has this disadvantage when compared with the hard 

 bast and libriform fibres, that its absolute strength is somewhat less; its bearing 

 capacity is only 10-12 kg. to the sq. mm. in cross-section. The limits of elasticity 

 of the collenchyma are also considerably less, but where hard bast or libriform cells 

 would be unsuitable, from the reasons stated above, collenchyma replaces it. It 

 cannot be said that hard bast and libriform fibres are more important than 

 collenchyma; each in its own way has an especial architectural value, and some- 

 times the one, sometimes the other, is the more advantageous. 



The hard hast, lihriform cells, and collenchyma which are comprehended under 

 the common term mechanical tissue are usually arranged in strands running 

 parallel to the long axis of the stem. If they were confined to the centre it 

 would be anything but a suitable arrangement, for an erect stem; they would 

 contribute almost nothing to the resistance to flexion as will be seen from the 

 following considerations. Let us imagine a horizontal, cylindrical stem resting 

 on solid supports at either end and loaded in the middle; it will bend downward.s 

 in proportion to the load laid on it, and in doing so the concave side will be 

 shortened and the convex side lengthened; the shortened side will be subjected 

 to compression and the elongated side to tension. These forces will be greatest 

 at the periphery, on the upper and under limiting surfaces, of the bent stem. The 

 opposed forces diminish towards the middle of the stem, and completely vanish at 

 the centre, therefore, in order that the stem should resist bending as much i\s 

 possible, it is obvious that the strengthening material is best applied when 

 wholly used in the form of flat plates where the forces are greatest. These 

 particular constructive pieces are known technically as Jiavges, and a flange is 

 fixed at either side of a beam which requires to be strengthened against flexion. 

 The mass lying between the two flanges is called the web, and the whole beam 

 so constructed is termed a girder. Fig. 177 ^ gives a diagrammatic representation 

 of such a girder in cross-section. The web may be composed of much softer 

 material than the flanges; it may consist of a lattice- or merely of a frame-work. 

 Where these girders are developed in plants, the web consists of vascular bundles 

 or of parenchymatous cells, while the flanges are always built up of meclianical 

 tissue. In flat, extended foliage-leaves the girders are fitted in so that their flanges 

 are parallel to the upper and lower surfaces of the leaf, but these leaves only resist 

 bending in one plane. This construction, which can be seen in the leaf-sections 

 given in figs. 86 ^ and 87 ^ (pp. 842-343), would be ill-adapted to stems. An erect 

 stem which is struck by the wind, sometimes from one side and sometimes from 

 another, must be strengthened indiflerently on every side, and in accordance 



