THE ROOT 115 



paper, it only seems to be dry ; in reality it is always 

 suffused with the liquid that washes its inner surface, 

 and this liquid was not simply water, but water slightly 

 acidified with acetic acid. Therefore the bladder is 

 moistened with an acid, and acids, as we know, dissolve 

 chalk. The chalk dissolves at every point of contact 

 with the moistened bladder, and this solution passes 

 through the bladder into the jar. All this happens 

 imperceptibly, and so it appears as if a dry solid body 

 passes in some incomprehensible way through a dry 

 bladder. We gather from this experiment that if only 

 cell -walls be moistened with an acid, solid bodies 

 dissolving in that acid can easily pass through them. 



Does not something of this kind happen in roots ? 

 In order to prove the possibility of such a phenomenon 

 it is necessary to show that the surface of roots gives 

 an acid reaction. For this purpose we have only to 

 apply to the root a piece of litmus paper, which chemists 

 use for detecting the presence of acids. Under the action 

 of acids the blue colour of this paper changes to red. 

 The root-tips do actually leave a red trace on the blue 

 paper. There are indications which point to the fact 

 that sometimes this acid is actually acetic acid which 

 we used in our last experiment. Moreover, the root, 

 like every other part of the plant, is continually breathing 

 and giving off carbonic acid. This can be proved by 

 an experiment similar to the one demonstrating the 

 respiration of germinating seeds. Now carbonic acid 

 dissolves many substances which are insoluble in water. 

 Here is, for instance, water into which phosphate of 

 lime in the form of a very fine powder is stirred ; two 

 very important nutrient substances, calcium and phos- 

 phorus, have thus been mixed with the water. I pass 

 through this water a stream of carbonic acid, and in a 

 short time the turbidity disappears the salt has 

 dissolved. 



So, then, roots have an acid reaction and, moreover, 



