PROGRESS IN PLANT PHYSIOLOGY 81 



for De Saussure to show this additional and important 

 part played by water in the plant's nutritive economy. 



De Saussure also showed that growth was impossible 

 without respiration, and that the more active growth 

 was, the more vigorous also was respiration. He found 

 that no growth took place and that nutrition was abnormal 

 in absence of minerals, and hence that these were not 

 accidental impurities but essential constituents of the 

 raw materials of the plant's food. His work contains 

 a large number of analyses of plants and of soils, and he 

 presents us with a long series of what might be termed 

 balance-sheets of the income and outgo in various plants, 

 and thus he arrived at a tolerably clear conception of 

 what was necessary for adequate plant nutrition. 



One very important conclusion he reached was that 

 the nitrogen in the plant's composition did not come from 

 the air but from the soil. He went astray somewhat in 

 determining the precise source of the nitrogen, for he 

 conceived it as being derived from animal and plant 

 waste or from ammoniacal compounds formed during 

 the decomposition of such waste. This was indeed the 

 first beginnings of the so-called " humus theory " that 

 retained its hold on the minds of plant physiologists for 

 many years, until it was finally disproved by Boussingault 

 about the middle of the century. 



There is one other point which De Saussure emphasised, 

 viz. that roots absorbed far more water than was required 

 by the plant for nutritive purposes, and that the salt solu- 

 tion taken up was extremely dilute; but his ignorance 

 of the law of osmosis prevented him from offering any 

 appropriate explanation of this phenomenon. He noticed, 

 however, that roots absorbed substances in solution 

 whether these substances were of service in nutrition or 

 not, thus contradicting the older view that roots had the 

 power of selecting from the soil just what they required. 



To sum up, it may be said that the chief merit of De 

 Saussure's work lies in the rigidly scientific methods he 



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