ABSORPTION OF WATER. 53 



air is usually lower at a high than at a low temperature, 

 the loss of water from the leaves is greater when the tempera- 

 ture of the air is higher. This drain upon the water in the 

 plant leads to an increased absorbent activity of the roots. 

 But although increase or diminution of transpiration produces 

 a corresponding change in the activity of absorption, yet the 

 two functions are not directly proportional. Nor does ab- 

 sorption necessarily depend upon transpiration, for. as we 

 shall see hereafter, absorption takes place with considerable 

 activity at times when the plant is not transpiring. This 

 point will be fully discussed in a subsequent lecture when 

 the subject of transpiration is being considered. 



We will now consider more particularly the absorption 

 of substances in solution, and, inasmuch as these substances 

 are usually inorganic chemical compounds, we may briefly 

 designate them as salts. We may very well begin by studying 

 the process of absorption as it takes place in the Yeast-plant 

 In the first place it is obvious that the substances absorbed 

 by it are dissolved in water; that the substances absorbed 

 are of such a nature that they can not only diffuse through 

 the cell-wall, but also pass through the primordial utricle ; 

 and further, that the continued absorption of any substance 

 depends upon the fact that the proportion of it in the 

 Pasteur's solution is greater than the proportion of it in the 

 cell-sap of the Yeast-plant, for this, we have already seen, 

 is a necessary condition of osmosis. In the case of a sub- 

 stance contained in the Pasteur's solution and used by the 

 Yeast as food, this inequality is maintained by the meta- 

 bolism of the organism. When a quantity of the substance 

 is absorbed, it undergoes chemical change : the effect of this 

 is to diminish the amount of the substance in the cell- 

 sap, and thereby to cause the absorption of a further supply 

 of it. 



The process of absorption is essentially the same in the 

 root-hairs of the higher plants. Only such substances are 

 taken up as are soluble and can pass through the cell-wall 

 and primordial utricle, and the continued absorption of any 

 substance depends upon the fact that the proportion of it in 



