CHAPTER IX. 



TBANSFER OF WATER THROUGH THE PLANT. 



686. WATER is a constituent of all active cells. The proto- 

 plasmic body of the celt possesses a marked affinity for it, and 

 up to a given point can abstract it from the ordinary surround- 

 ings, but under certain conditions releases it again. If a water- 

 plant in full activity is removed from water and exposed to the 

 air, it speedily loses by evaporation a considerable part of its 

 constituent water, and shows the effect of this loss by a col- 

 lapsing of its cell- walls and by a withering of all its parts. But 

 if only a small portion of the plant is lifted above the surface 

 of the water, the loss which takes place will be partially sup- 

 plied by transfer through the cells remaining submerged. Two 

 points are made clear by this simple experiment : (1) evapora- 

 tion goes on with great rapidity from the exposed surface of the 

 plant ; (2) only a part of the loss of water can be made good by 

 transference from submerged portions. 



687. Comparison of the structure of a water-plant with that 

 of an ordinary plant adapted to growth in the air shows that 

 the surface of the latter is such as to prevent very rapid evapo- 

 ration, and also that the loss caused by the evaporation can be 

 made good if the lower part of the plant remains in contact with 

 water. In other words, the plant (1) has a surface which protects 

 it against too great loss of water ; and (2) is provided with a 

 system by which the needed supply of water can be replenished. 



688. But it is not alone by evaporation from the surface that 

 water is consumed by the plant. Wherever growth goes on or 

 work is done, water is consumed, and a fresh supply is required. 

 The question of the transfer of water is therefore a general one. 



SOME OF THE RELATIONS OF WATER TO TISSUES. 



689. The cell-wall which separates the cavity of one cell from 

 that of its neighbor is a permeable membrane. According to 

 the hypothesis of Nageli (see 588), it is composed of solid par- 

 ticles (micellae), each of which is enveloped in an adherent film 



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