52 LECTURE IV. 



ment in solutions containing from to 2 per thousand of 

 inorganic salts. The branching of roots is affected by the 

 proportion of water in the soil, the number of branches 

 being greater, according to Unger, when the soil is moist 

 than when it is dry; in the latter case the development 

 of root-hairs is more considerable. It is also a well-known 

 fact that roots always tend to grow towards moisture, 

 a fact which cannot be satisfactorily explained at present. 

 An interesting experiment of Nobbe's has brought another 

 fact of this kind to light. He cultivated Maize and Clover 

 plants in pots containing layers of soil of which some had been 

 previously soaked in solutions of nutritious substances, and 

 he found that the roots branched much more freely in the 

 layers which had been thus treated than in the others. 



Of the external conditions which materially affect the 

 absorption of water by roots an important one is the tem- 

 perature of the medium in which they are. From the ex- 

 periments of Sachs we learn that roots absorb water more 

 actively at a higher than at a lower temperature. For this 

 function, as for all others, there is an optimum temperature 

 at which it is performed with the greatest activity, and above 

 or below which the activity diminishes. 



But the absorption of water by roots is not affected only 

 by the temperature of the medium in which they themselves 

 may be, but also by that of the medium which surrounds 

 the other parts of the plants. Thus roots absorb with greater 

 activity when the air is at a high than when it is at a low 

 temperature. 



Vesque has found that, under certain conditions, a sudden rise in the 

 temperature of the soil, but more especially in that of the air, has the 

 effect of temporarily diminishing the absorption, and a sudden fall of 

 increasing it. The explanation of this will be given in connexion with 

 the subject of transpiration (p. 118). 



The explanation of the effect of the temperature of the 

 air upon the absorbent activity of the roots depends upon 

 the fact that, under ordinary conditions, the leaves of a 

 plant exhale a considerable quantity of watery vapour 

 (Transpiration). Inasmuch as the degree of humidity of the 



