324 The Physiology of Plants BOOK in 



time of the opening of the period with which the present 

 volume is concerned. The results which we have already 

 quoted were arrived at and published a few years before ; 

 so that in 1860 the new ideas had begun to take hold of 

 men's minds and were finding general acceptance. In 1860 

 and 1861 we find him giving exact evidence, based on 

 quantitative estimations, that the green plant needs nitrates 

 in the soil if it is to flourish. The experiments, very 

 striking ones, may be quoted as setting out the position 

 at the opening of our period. 



Boussingault made three series of cultures of Helianthus 

 argophyllus ; the first in sand without any minerals con- 

 taining combined nitrogen ; the second in sand, to which 

 were added the materials of the ash of the plant and some 

 nitrate of potassium ; the third differing from the second 

 only in that carbonate of potassium replaced the nitrate. 

 After eighty-six days the plants of the first and third series 

 showed no increase in nitrogen beyond a trace that might 

 well have been derived from a little ammonia formed in 

 the air, while those of the second series had gained a con- 

 siderable quantity. The development of the plants of the 

 first and third series was only about one-sixth of that of 

 the second. 



While Boussingault was occupied with these problems 

 in France, Lawes and Gilbert were giving them assiduous 

 and careful examination at the Rothamstead experimental 

 station in England. Working on similar lines they arrived 

 at results which were confirmatory of those of the great 

 French physiologist. A very comprehensive paper, in pre- 

 paring which Pugh was associated with Lawes and Gilbert, 

 was published in 1861. 



The question appeared to be definitely settled by these 

 researches, and there seemed no doubt that the atmosphere 

 is to be altogether put on one side as a source of supply of 

 nitrogen. So for several years matters rested, when certain 



