6Q SOME ADAPTATION OF WATER PLANTS, ETC. 



mis is not differentiated in this manner, as there is no danger to 

 them, by very reason of their environment, of any loss of the 

 precious fluid ; but as the latter, with its contained food-salts, 

 enters them through their whole surface, it is necessary for their 

 epidermis to have absorptive powers. Consequently, there is no 

 thickening or cuticularisation of the submerged external walls, the 

 cells are conterminous everywhere, leaving no openings and 

 forming an excellent absorptive surface, no organs being needed 

 either for the conduction or transpiration of water. 



The amount of transpiration also varies with the temperature 

 and climate. In very hot countries where the loss of water by 

 evaporation is correspondingly great, the cuticle is markedly evi- 

 dent, consisting of several layers of cells, this formation being well 

 seen in J^i'cus e/asfica, the India-rubber plant. In plants growing 

 in temperate regions in damp places, the quantity of this tissue is 

 considerably less, and where it is remarkably so transpiration 

 takes place very readily. For this reason, trees like willows are 

 planted in bogs to dry up the moisture in them. 



Another specialisation of the cellulose is to be found in sea- 

 weeds. In them there is no epidermis, but the cells towards the 

 periphery are crowded together, and the cell-walls, generally, 

 become mucilaginous. When the tide recedes and leaves the 

 algae exposed, tbe mucilage hardens, giving them the horny 

 appearance with which we are all familiar, when by any mischance 

 they are withdrawn from their native element. The hardened 

 mucilage prevents dessication of the protoplasm or living principle 

 of the cells, and thus preserves the plant alive and intact. On 

 being again covered with water, the mucilage readily absorbs the 

 liquid, and immediately becomes softened and absorptive again, 

 and the plant is restored to full vigour of life, having suffered no 

 injury from its temporary embarrassment. 



IV. — There are present in water, in addition to the carbon 

 dioxide already spoken of, other salts that are essential to the 

 well-being of the plant which are in a state of solution, and which 

 enter the organism in the same manner. It might, then, be 

 thought of advantage to a plant to fix itself in fast-running water, 

 where a constant, ever-changing, and plentiful food supply would 

 be presented to it. This is, however, far from being the case, and 



