THE RESPIRATION' OF PLANTS. 67 



oping their vitality, and promoting the growth of the organs which 

 discharge that function. Hence it would appear that, for plants, as 

 for animals, an abundant absorption of pure air, rich in oxygen, is 

 the essential condition of a strong, vigorous constitution. 



But these individual or specific differences have no importance as 

 regards our problem. The point for us to consider is, not the abso- 

 lute proportion of a given element, but rather the relative modifica- 

 tions which the initial proportion undergoes during the life of the leaf. 

 Let us see what the tables have to say on this point : 



1. During the growth of the leaves, the relative proportion of nitro- 

 genized matter in their tissue grows rapidly less. It is at the maximum 

 just when these organs are breaking out of the bud, and it goes on 

 decreasing thenceforward till about the beginning of July, when the 

 fruit of the lilac has been formed. From that time on, the quantity of 

 nitrogenized matters varies but little, though it seems to gain a little 

 in leaves approaching maturity. Finally, it is at the minimum when 

 the process of vegetation is complete. 



At the moment of their falling, we find in lilac-leaves only about 

 one-third the amount of nitrogenized matters they contained at the 

 outset. In maple-leaves the amount is relatively greater, but the 

 difference is not very important. 



2. If, now, we look at these analyses with respect to the amount of 

 carbonaceous matters, we find that the latter rapidly increases from 

 the moment when the leaves start from the bud, down to the time 

 when they have attained their greatest size, i. e., when they have 

 reached the adult age. As regards the lilac, this is the case when the 

 flowers are ready to expand. The carbonaceous matters thenceforth 

 gain less notably till September ; but then we perceive a sensible 

 decrease, especially as regards the maple. Finally, they attain the 

 maximum at the time of falling from the tree, this rise being due to 

 the disappearance of a notable amount of nitrogenized substances. 



3. The ash, too, increases rapidly down to June, but then it grows 

 less pronounced. There is relatively a greater amount of mineral 

 matter in the faded leaves of the maple than in those of the lilac. 

 The latter, at the close of their life, show a slight diminution, which 

 is perhaps accidental, in mineral salts : being more tender than maple- 

 leaves, they probably lose a little of their soluble salts under the action 

 of rains. 



We have only to compare the mature leaves of the maple with 

 those of the lilac, in order to see that the former must contain more 

 fixed salts than the latter : the fibres which traverse them are thicker, 

 stronger, and more numerous, than in the leaves of the lilac, and hence 

 they are richer in silica and salts of lime. 



The ashes of these two trees differ very widely from one another. 

 Even in the same species the quantity of the ash, like that of the 

 nitrogenized matter, differs according to the surroundings, the age of 



