66 HISTORY OF BOTANY 



sunlight, and so on. Plants obtain most of their con- 

 stituent juices from the soil by their roots and their 

 carbon from the atmosphere whence they absorb air. 

 This they elaborate in their leaves, separating from it the 

 carbon they require for their own nourishment and 

 throwing out the remainder which is useless to them 

 but useful to animals, who in their turn take from the 

 air, in the act of respiration, what they want and throw 

 out the residue hurtful to them, but rendered serviceable 

 once more to the plant. The oxygen yielded by the 

 plant is elaborated by a kind of vital motion carried on 

 in the leaf and kept up by the influence of sunlight." 



From this summary I think you will at once recognise 

 how great is the advance made by Ingen-Housz on the 

 knowledge of the physiology of nutrition as described by 

 Hales. No further progress was possible until chemists 

 had established clearly the composition of the atmosphere, 

 and Ingen-Housz is handicapped by the lack of this 

 essential information. Still even in the Experiments he 

 gives a fairly correct account of the gaseous interchange 

 between the plant and the atmosphere. It is quite 

 apparent, however, that to him the plant world is entirely 

 subservient to the animal world and primarily a physio- 

 logical apparatus for purifying the air to make it fit for 

 animal use. He does not seem to realise that the whole 

 of the animal world is dependent in the long run on the 

 plant world for its nutriment. 



In 1796 Ingen-Housz produced another work entitled 

 On the Nutrition of Plants and the Fruitfulness of the 

 Earth, in which he shows that he is now armed with the 

 knowledge he had acquired of the new chemistry founded 

 by Lavoisier, a knowledge he confesses he did not possess 

 when he wrote the Experiments. He knew now that 

 carbon dioxide was a compound of carbon and oxygen, 

 and this at once enabled him to grasp the significance of 

 the gaseous interchanges taking place between the green 

 leaf and the air in sunlight. His new interpretation of 



