68 COMPENDIUM OF GENERAL BOTANY. 



ing tissues), and which is also peripherally located, must make 

 suitable concessions to the mechanical tissues as to position. This 

 is what actually takes place. 



IY. Theoretically the strength of a given support would depend 

 only upon the magnitude of its cross-section. It is, however, evi- 

 dent that six silk threads which are about one cm. apart and so 

 placed that one is central and the other five peripheral, are in danger 

 of being torn by some pulling force, because tension on them is 

 very apt to be unequal. This unequal tension may be counter- 

 acted to a considerable degree by bringing the threads in contact so 

 that tension will act on all of them at the same time. The consid- 

 eration of various roots which are also subject to pulling tensions 

 teaches that a central arrangement of mechanically resisting elements 

 is the intended plan of structure. According to a similar principle, 

 the centripetal tendency of mechanical elements is also found in 

 such structures as tendrils, plant-organs in rapidly flowing water, 

 stems of climbing plants, and stalks of fruits. In rhizomes, which 

 morphology shows to be stems, and in running stems (creepers), 

 the anatomist finds a tendency on the part of peripheral mechanical 

 elements to assume a more central position, hence from a morpho- 

 logical point of view they have a resemblance to roots. The large 

 number of phenomena belonging to the domain of plant mechanics, 

 which SCHWENDENER has so faithfully studied, cannot be fully dis- 

 cussed here. Two things, however, remain to be mentioned. Cer- 

 tain rhizomes living in very moist soil have the outer parenchyma 

 supplied with air-spaces, since they are mostly surrounded by water ; 

 collapsing of this tissue is prevented by a thin peripheral layer of 

 bast-cells. The supporting roots mentioned above (Zea Mays) show 

 almost an equal distribution of mechanical elements, so that these 

 roots are midway between typical supporting organs and flexible 

 organs. 



In such flexible expanded organs as the leaves the mechanical 

 cell-complexes are in two layers, one for each surface ; this is in 

 accordance with the mechanical principles explained above (coin- 

 pare Fig. 41). 



It must not be forgotten that thin-walled turgescent tissues 

 represent a more or less firm substance, and in suitable positions 

 (for example, at opposite sides of a vascular cylinder, or as filling 

 material around the vascular bundles) it very materially assists in 

 increasing the flexibility or firmness of plant-organs. 



