42 EVOLUTION AND GROWTH. 



I conceive that it is by such a method as this, that the genera, 

 phenomena of vegetable respiration in plants still connected with the 

 soil ought to be studied. 



The experiments which I have now related must satisfy every 

 one, that the leaves of living plants actually assimilate the carbon 

 which occurs in our atmosphere in the state of carbonic acid ; they 

 also explain the well-known fact that plants thrive better in air that 

 is in motion, and frequently renewed, than in a perfect calm. 



From all we have seen up to this time, then, w^e feel authorized 

 to conclude that the greater proportion if not the whole of the carbon 

 which enters into the constitution of vegetables is derived from the 

 carbonic acid of the atmosphere. The experiments cited, show- 

 how the vital force acts at first on the oxygen of the air during ger- 

 mination, and next upon its carbonic acid during vegetation properly 

 so called. But in none of the experiments which have been quoted, 

 have we seen any thing which could lead us to suspect that the 

 azote of the atmosphere was absorbed in sensible quantity. 



It is true, indeed, that at one time Priestley, and after him, Tn- 

 genhousz, thought that they had observed an absorption of azote 

 during the growth of plants in confined atmospheres. But the ex- 

 periments which have been since performed by Saussure have not 

 confirmed their conclusions upon this point. Saussure even thought 

 that he had perceived a slight exhalation of azote. 



Nevertheless, the presence of azote in vegetables being incon- 

 testable, and the assimilation of this principle during their growth 

 being in some sort demonstrated by the fact that seeds are multiplied, 

 physiologists were led to imagine that the azote was derived from 

 ihe soil. And in nature, indeed, the growth of a plant does not take 

 place at the sole c«)st of water and the atmosphere. The roots 

 w^hich attach it to the earth there also find elements of nutrition. 

 In ordinary circumstances the growth of a plant takes place by the 

 simultaneous concurrence of the food which the roots encounter in 

 the ground, and that which the leaves abstract from the gaseous 

 elements of the air. As it is further acknowledged that the food 

 which is supplied by the soil is for the most part azotized, manures 

 have therefore been regarded as the principal and even as the ex- 

 clusive source of the azote which is met with in vegetables. The 

 observations of Hermbstadt, in showing that the grain which was 

 grown under the influence of the most highly azotized manures 

 contained the largest quantity of gluten, gave a certain force to this 

 view. Nevertheless, there are facts well established in agriculture 

 which induce us to think that in many cases vegetables find in the 

 atmosphere a part of the azote which is necessary to their con- 

 stitution. 



The majority of crops exhaust the soil ; but there are still some 

 n'hich render it more fertile. We shall see, by and by, when treat- 

 ing of the rotation of crops, that if, after having cut a field of trefoil 

 once, the second crop be ploughed down, new fertility is communicated 

 to the ground, in spite of the considerable mass of forage which had 

 previously been taken from it. It appears therefore evident, that 



