Page:Popular Science Monthly Volume 84.djvu/566

562 and have a sufficient underground supply of moisture. In the matter of soil requirements, therefore, it is easily suited, and it is hardly necessary to add that this is a distinct asset on the side of the perpetuation of the species.

Just as is the case with other organisms in both plant and animal kingdoms, all our tree species possess an average age limit. Some of the individuals of the species live longer, and others die out before the limit is attained, but to every species one might assign an average length of life. This age limit differs, of course, with different species: for example, some of the oaks are notably longer lived than such species as the aspen or the gray birch. Some of the causes of these differences in longevity are obscure, and this is not the place for their discussion. What we wish to point out is that trees as well as animals, vary in their age limits.

Now, a tree, on reaching maturity, begins to reproduce, that is it forms seeds, which, if they find conditions suitable for them, develop into individuals like their parent. I have here in my yard a silver maple, the seed of which I planted in 1904. Two years ago, in 1912, when it was eight years old, and about ten feet high, it bore a few blossoms and seeds. Last year more seeds were produced than the year before, and this year it is loaded with blossoms. Although still a small tree, not yet ten years old, it is arriving at maturity, and is able to reproduce itself. It should continue to do this until the end of its life. Of course under modern city conditions, where, among other disturbing elements, the ubiquitous lawn mower can always be counted on to do its deadly work, it is a question how many, if any, of its descendants will survive. But, nevertheless, here or anywhere, the chances of reproducing its kind depend ultimately on the number of seeds it bears, and this number, again, depends directly on the length of its life.

It is clear then that with two species with differing age limits, other things being equal, the species which is longer lived would have the better chance to perpetuate its kind.

When we consider the genus Castanea, we find it especially favored in this respect, for it has long been noted for its longevity. The following extract from Sargent's "Silva of North America" is of interest in this connection:

Further on, writing of trees on Mt. Ætna, in Sicily, Sargent says: