62 SOME ADAPTATION OF WATER-PLANTS 



porting pillar of a bridge, which, in accordance with well-known 

 mechanical principles, is hollowed in order to throw all the strength 

 to the outside, the better to resist any lateral strain, and which 

 may be filled with cement in order to sustain a more crushing 

 weight. But from the very nature of its environment, a hydro- 

 phyte is protected from winds, and hence from lateral strains. Its 

 parts float freely, and rise and fall with the waters or move hither 

 and thither in obedience to currents. The only force, therefore, 

 that can be brought to bear upon the plant in this situation is a 

 puUing one. In still water, where there are no currents, and the 

 only turmoil is that temporarily caused by the movement of some 

 animal organism or by the breeze, the innate recuperative power 

 and cohesive force of the cell-walls of the freely floating portions 

 is sufficient protection for the plant ; but in running water some- 

 thing more than this is needed, and the tissues become flexible 

 and tougher and their walls thickened. 



In any case as compared with land-plants, the bundles become 

 fewer in number, and have a tendency to assume an axial position 

 (see T.S. of Villarsia), Neither do the component cells become 

 lignified to the same extent, as the absence of any great strain 

 relieves the plant from the necessity of making what would, under 

 the circumstances, be useless tissue. As a further illustration of 

 the mechanical importance of the central grouping of the fibro- 

 vascular bundles, take the analogous case of a root. A root, apart 

 from the pressure exerted by the superincumbent soil, is liable 

 only to the pulling or longitudinal strain caused by the bending 

 and swaying of the stem, in these respects resembling a hydrophyte. 

 The strain to be resisted, however, is very much greater, and the 

 structural arrangement best adapted to meet it is the central 

 grouping of the resisting forces, as has been proved in practice by 

 man, who uses the same means to obtain the same ends. Under 

 parallel circumstances, the aquatic stem and a root develop an 

 approximately similar structure, so far, at least, as the grouping of 

 the bundles is concerned. 



In addition to the adaptation just mentioned, water-plants have, 

 in various parts of their other tissues, in leaf and stem and petiole, 

 numerous air-spaces which are not to be confounded with similar 

 cavities in the hollow stems of Equisetum grasses and UrnbelUferce. 



