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It seems plain enough - although it isn't at all plain -- how a tree, once it has reached treehood, can keep on being a tree, growing more leaves, twigs, branches, bark, roots, and so on -- but how these things and its flowers and the standard pattern of the family shape it takes are all determined beforehand in the simple little seed out of which it comes -- that's what beats me! I said as much several years ago, (In The Strange Adventures of a Pebble.) and, really, there isn't much more to say. As one writer puts it: (Trees, Shrubs, and Vines, Parkhurst.) In the seed are the sealed orders for the whole plant's career, from the Sequoia that lives five thousand years to the climbing vine that dies in six months and the grass which to-day is and to-morrow is cast into the oven. Another, a very learned man indeed, as you can see from the big words he uses, says: A seed is of complex structure and multiple origin. The seed coat belongs to the parent sporophyte. The endosperm is gametrophytic in origin in the gymnosperms, while in the angiosperms it is a unique body in a sense homologous with the new sporophyte. The seed may thus include three generations in its constitution. It is, therefore, simply a young plant in which development has been arrested and which at this stage has been cast loose from the parent, enclosed in a protective covering, and well stocked with food. In other words, for this reason and for that, a seed is "simply" a youngster whose parent has, in some mysterious way, furnished it with a full set of working drawings, showing just what style of a plant it's to build itself into, put on its cloak, given it a basket of food, and said: "Good-by and good luck!" But, little as we know about the "how" of it, the fact that a tree can reproduce itself is very important, not only to foresters, but to the whole world; otherwise it wouldn't be very long before every country would be as barren as the Sahara Desert; barren, not only of trees, but of everything else -- including us! AS IN THE PHARAOH'S DREAM Some seed is produced every year, but, as it was with the wheat in the Pharaoh's dream, there are years of plenty and years of famine. The years of heavy seed-production vary in frequency with different species. They also depend on weather, climate, and soil. Light, next to appropriate soil, is most important in the production of seed, just as it is in the growth of the tree; so trees in the open, where they get plenty of light, produce a larger amount of seed than those that are crowded, and they begin seeding at a much earlier age. Twenty-five years is the average age at which open-grown trees begin bearing, while in crowded communities it is forty. Middle-aged trees produce better than very young or very old ones. Recalling what was said in the first sentence about light, you will understand why trees in the forest bear chiefly at the top while those in the open have seeds at the ends of practically all the branches. And, in the forest, trees that are obliged to spend their lives in the shadows of taller neighbors, rarely bear any seeds at all. The genial warmth of the sun is also very important; so, other things being equal, trees bear better toward the southern part of their range than toward the northern border. Seedlings from seed gathered in a southern climate are apt to succumb when set out in colder regions. Seedlings from Black Walnuts gathered in Missouri are killed by the spring frosts in Minnesota. Some species of trees bear seeds at a comparatively early age, but only a small proportion of such seeds will grow. Seed production is at its best after a tree has got its growth, so far as height is concerned; for trees, like people, stop growing taller, in course of time, and begin putting on girth. When they do finally get to the time when they begin reproducing after their kind, trees keep it up to a very old age, and it is not uncommon to find them making heavy crops a year or two before they die; but, as I have said, trees in middle life, as a rule, bear the best seed. |
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