424 ARRANGEMENTS FOR RETAINING THE POSITION ASSUMED. 



AERANGEMENTS FOR RETAINING THE POSITION ASSUMED. 



When the green tissues of plants have once assumed the position most beneficial 

 to them, they must be kept as long as they can be in that position, and any further 

 alteration must be as far as possible avoided. The displacements, curvatures, and 

 extensions described in the preceding pages, representing a struggle for the best 

 arrangement of the green tissue for light, must not be restricted; whilst distortion, 

 folding, and rupturing of the chlorophyll -containing tissues, which would be 

 synonymous with the destruction of the portion in question, must obviously 

 be warded off. 



In the depths of still water, at the bottom of pools, ponds, and lakes, an 

 alteration of the position assumed by the fully-developed plants in consequence of 

 an external stimulus occurs but seldom; and although currents and eddies are set 

 up in the water by passing aquatic animals, and temporary oscillations caused in the 

 water-plants, these quickly subside, and the agitated portions return forthwith to 

 their original position, having suffered no injury. In aquatic plants of this kind 

 there are no special contrivances for strengthening the individual organs, and in 

 particular no contrivances for protecting the green tissue from rupture and 

 crushing. The small amount of strength and elasticity of the cell-walls suffices to 

 withstand the thrusts, and pulls, and the pressures which make themselves felt in 

 the depths of the water, and to restore the temporarily displaced green portions to 

 their right position. Firm woody cells, and strands of elastic bast-fibres, which 

 play such an important part in the aerial portions of plants, are wanting here. 

 Woody plants neither grow in the sea, nor in fresh water. Aquatic plants, indeed, 

 quickly collapse, in consequence of the absence of wood and bast, when brought 

 from the depths into the air; the leaves collapse of their own weight, and sink 

 flaccidly on to the substratum. They are able to retain an erect position in 

 the water, because a portion of their tissue is penetrated by comparatively large air 

 spaces, by which means their specific gravity, compared with that of the water, 

 becomes much diminished. If aquatic plants were not firmly attached to the sand 

 and slime, or submerged rocks, they would rise to the surface and fioat there. 

 But as they are fixed in the depths, the air spaces within the green tissue of the 

 leaves or stalks bearing the leaves cause these organs to remain erect as if 

 suspended in the water. 



Plants growing in running water, and such as are exposed to the lapping of 

 the waves on the shore, are indeed subjected to a severer proof of their firmness 

 and tenacity. Thus many of them, e.g. sea-wracks on the sea-coast, the long- 

 leaved pondweeds in the quick-flowing mountain streams, and the Podostemaces 

 in the rushing torrents and waterfalls in tropical regions, are actually swayed 

 hither and thither and continually shaken, and accordingly due allowance must 

 be made in their construction for this circumstance of their habitat. The tissue 

 of these plants is much tougher than that of the Characese, of the Naiadaceae, 



