186 MECHANICAL SYSTEM 



are much broader, and also project inwards to a far greater extent, 

 in the inflexible region. In the inextensible underground portion 

 the average cross-section of a girder only amounts to between one- 

 third and one-fourth of its area in the upper inflexible region ; the 

 inner flanges of the main girders, on the other hand, show a distinct 

 increase in thickness, while the external fibrous sheaths of the large 

 mestome-bundles are so stout that they can scarcely possess a purely 

 local significance. 



With the aforesaid subterranean organ may be associated the 

 stems of twining plants and of climbers in general, since these are 

 also continually subjected to longitudinal tensions. In the case of 

 twining stems, the tensions in question arise from the growth in 

 thickness of the supporting structure, and from the flexures produced 

 in the latter by the action of the wind. In other types of climbers 

 allowance must also be made for the fact that the different points at 

 which a support is grasped may subsequently move further apart ; 

 further, more or less extensive regions of a climbing shoot may 

 temporarily or permanently assume a pendent position. 10J In accord- 

 ance with these peculiar mechanical conditions, the mechanical system 

 of climbing stems displays a centripetal tendency, which is sometimes 

 merely indicated by a sclerotic development of the pith (l\vmus and 

 Dioscorea), while in other cases it may lead to the centralisation of the 

 mechanical strands themselves. Thus in certain Monocotvledonous 

 climbers such as Calamus Rotang and species of Carludovica even 

 the innermost vascular bundles of the stem are furnished with stout 

 fibrous sheaths ; again, in the stems of certain Pipekaceae {Piper 

 and Mdcropiper) a mechanical cylinder is interpolated between the 

 pith and the peripheral circle of vascular bundles. 



All submerged plants, whether they inhabit running or stagnant 

 water, must have more or less inextensible stems. A special interest 

 attaches in this connection to the. genus Potamogeton, because of the 

 various habitats of the different species. Schwendener has demon- 

 strated, with the help of elaborate arguments, that the species which 

 are restricted to stagnant or slowdy-tlowing water (P. crispus, P. 

 densus, P. pectinatus) develop no specialised mechanical cells, either 

 in the cortex, or in the central mestome-cylinder, the latter being 

 itself able to cope successfully with the very slight demands that are 

 made upon the tensile strength of the stem. Those species, on the 

 other hand, which are adapted for life in rapidly streaming water 

 (P. lanceolatvs, P. hngifolius, P. compressus, etc.), not only exhibit a 

 corresponding reinforcement of the central cylinder by mechanical 

 elements, but are in addition provided with a number of scattered 

 fibrous strands embedded in the parenchymatous mesh-work of the 



