194 HISTORY OF BOTANY 



factors enter in, and the pressures developed are much 

 in excess of those demanded by transpiration. 



" Finally, it has been shown that the stored energy 

 set free by respiration in leaves is quite sufficient to do the 

 work of secretion against the resistance of the transpira- 

 tion stream ; while, when the vapour pressure of water 

 in the surrounding space is low, and when evapora- 

 tion is doing the work of raising the sap, the expenditure 

 of energy in this process will reduce the quantity of 

 water evaporated only by an imperceptible amount." 



THE ABSORPTION OF MINERALS 



The absorption of water by the roots and its transport 

 to the leaves naturally leads me to speak of the nature 

 of the minerals dissolved in it and carried by it in its 

 upward passage. There was no dearth of papers on the 

 subject of plant ash and the functions of the ash con- 

 stituents during the latter half of the nineteenth century, 

 but it is astonishing how little was added to our knowledge 

 in spite of all the labour expended. 



Physiologists started from De Saussure's law, viz. 

 that for every salt capable of absorption there was a 

 certain concentration at which the root hair took in the 

 solution as it was presented to it ; if the concentration 

 exceeded that optimum the plant absorbed less salt 

 and more water. Subsequent investigators, however, 

 soon discovered that the law was by no means of universal 

 applicability. It was found that when a plant was 

 cultivated in a medium composed of many salts in 

 different proportions, it did not absorb them equally 

 and indifferently, and that roots of different plants grown 

 in the same soil took up the salts in different proportions. 

 This led to the belief that plants had a power of selection, 

 both qualitative and quantitative. But this view also 

 was somewhat discounted by the discovery that plants 

 were able to absorb not only salts of no service 



