of its weight which the air can not carry; and it so constructs certain parts, for instance the leaves, of nearly all but the pines and their allies, that their form is best fitted for floating. The leaves are the organs in which most food is made. Their efficiency depends upon the amount of light which they can absorb, and they will evidently absorb most light if they are flat and placed at right angles to the rays as they come from the sun. This may be the main reason for the expanded form of the leaves, and it is the only reason which has been proved by experiment. But it is evident that a leaf is buoyed up more strongly and, therefore, requires less mechanical support if it is flat and more 01 less horizontal than if it were vertical or if it were cylindrical or cubical. Comparing weight for weight, we find more mechanical tissue in the pine-needle than in the flat leaf. And we find no such mechanical tissues even in the largest and longest submersed aquatics, some of which are as long as trees are tall.
The amount of mechanically strengthening tissue in a part or a plant has been proved by experiment to depend upon the amount of mechanical strain to which it is exposed. Garden plants which ordinarily carry the weight of their branches will be mechanically much weaker if supported on trellises. Conversely climbers and prostrate plants, if subjected to mechanical pull, will develop strengthening tissues which they ordinarily do not form. In these cases, the so-called inherited tendency to form or not to form mechanically strengthening tissues is so promptly overcome in the individuals experimented upon as to suggest some doubt whether there is such a tendency at all, whether the structure and behavior of living things is not more due to the influence of their surroundings than to inheritance.
We may conclude, then, that the presence in erect land plants of mechanically supporting tissues which are never found in submersed aquatics is not mere coincidence. The difference in the mechanical tissues of these plants is due, not to the differences in their places in any scheme of classification or to their degree of evolution, but to the differences in the buoyancy of air and water. Aquatic plants do develop mechanical tissues, but they resist the pullings, bendings and blunt blows which the waves give. These tissues can not support much weight.
The strength of the submersed aquatic will vary greatly according as it is a floating or an attached organism. All submersed aquatics which are unattached are mechanically weak and they are usually small, whereas those which are attached must develop a certain amount of mechanical strength to resist the tugging of the free parts against the holdfasts. Compare, for instance, Spirogyra and fresh-water Cladophora, plants of somewhat similar size, structure and situation. A Cladophora filament will break only under a much stronger pull than
