PREVENTION OF DEFORMATION 181 



mechanical tissue must be strongly developed as a " tension-flange " on 

 the concave side of the coiled region, whereas turgescent parenchyma 

 often provides a sufficiently effective " compression-flange " on the 

 convex side. In Cyclanthera explodens, for example (Fig. 70), the 

 tension-Mange consists of a broad ribbon-shaped mass of fibrous tissue 

 with slightly incurved edges; this is supplemented by three vascular 

 bundles, which abut against its inner side. On the convex side there 

 are two smaller vascular bundles, one near either edge, with large 

 fibrous sheaths ; these represent the compression-flanges of the tendril, 

 which is thus dorsiventral from the first, as in all CUCURBITACEAE. 

 The tendrils of Passiflora and Biynonia are at first radially symmetrical, 

 but subsequently become dorsiventral, owing to the fact that the 

 secondary woody cylinder develops much more strongly on the concave 

 side. 



3. Mechanical arrangements for the prevention of deformation. 



When a cylindrical organ is bent, its normally circular cross-section 

 tends to become elliptical. This fact can be easily verified by 

 bending a piece of india-rubber tubing. Similarly, when a foliage- 

 leaf or other flattened organ bends, the two faces tend to approach 

 one another. Bending thus invariably subjects the cross-sectional 

 outline of an organ to a certain amount of deformation ; it is incum- 

 bent upon every plant to guard as far as possible against such 

 distortions, which are decidedly injurious. Any alteration of the 

 cross-sectional outline of an organ entails corresponding changes 

 in the relative positions of its mechanical constituents ; but a 

 mechanical system is reliable only so long as its peripheral girders 

 and other components preserve certain fixed relative positions. It 

 must also be kept in mind that every deformation of the cross-section 

 involves radial distensions and compressions, which may react so 

 severely upon particular tissues as to disturb their normal activities, 

 or which may appreciably diminish the cohesion of the entire organ. 

 It is evident, for example, that undue compression must seriously 

 interfere with the functions of the conducting tissues, while excessive 

 distension may easily lead to the disorganisation of loosely constructed 

 tissues, such as those comprised in the photosynthetic system. 



Special stiffening arrangements are particularly necessary in the 

 case of organs which are pervaded by wide air-passages. The richly 

 branched cellular trabeculae that intersect the air-passages of some 

 Cyperaceae especially in the transverse direction collectively form- 

 ing a very elegant system of internal buttresses, are of this nature; 

 they attain their most remarkable development in certain species of 

 Scirpus. A similar mechanical effect is produced by the subdivision 



