Il6 LECTURE VII. 



enquiry as to whether or not the absorbent activity of the 

 roots, or in other words, the amount of water in a plant, affects 

 its transpiration. It appears that this is not the case. Von 

 Hohnel has shewn the transpiration of the leaves is not affected 

 by variations in the quantity of water which they contain. 

 Still it cannot be doubted that if the cells of the leaves have 

 lost a large proportion of their water in consequence of active 

 transpiration and of inadequate supply, the water remaining in 

 their walls and in their protoplasm is retained with consider- 

 able force, and that consequently a great resistance will be 

 offered to its evaporation. 



Now as to the effect of transpiration upon the absorption 

 of dissolved salts by the roots. It was pointed out, when we 

 were considering the function of absorption, that only very 

 dilute solutions of salts can be taken up by the roots ; as a 

 consequence it is necessary that relatively large quantities 

 of water should be absorbed in order that the plant may 

 be supplied with the salts which are of importance in its 

 nutrition. The active absorption which is hereby involved is 

 promoted by transpiration, and it is by transpiration also that 

 the excess of water is got rid of. This is the course of reason- 

 ing by which many authors (most recently Wiesner) have 

 come to attribute to the function of transpiration an important 

 physiological significance with regard to the nutrition of 

 plants. The only direct evidence bearing upon the subject is 

 afforded by Schlosing's observation that a Tobacco-plant grown 

 under a bell-jar yielded a smaller proportion of ash than a 

 similar plant grown under ordinary conditions, the proportion 

 of silica in the ash being especially small. This evidence 

 cannot, however, be regarded as conclusive, inasmuch as the 

 conditions were such as to affect many of the functions of the 

 plant : and further, we know that many plants thrive best in 

 an atmosphere which is nearly if not quite saturated with 

 moisture. Still, there is reason to believe that transpiration 

 is of some importance in ensuring an adequate absorption of 

 salts by most plants. 



With regard to the distribution by the transpiration-current 

 of substances absorbed by the roots, there is a considerable 



