Page:Popular Science Monthly Volume 6.djvu/77

Rh 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 absolute proportion of a given element, but rather the relative modifications 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 nitrogenized 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