Page:Popular Science Monthly Volume 63.djvu/248

244 In the ferns and their allies, and in the grasses, the tissues mechanically supporting the parts above ground are combined into what may be called an external skeleton. This is distinct from the conducting tissues. It forms a cylinder close under the epidermis and enclosing the conducting and storing tissues. Each strand of conducting tissue may also be inclosed in a strengthening cylinder. This kind of skeleton is strong for the weight and amount of material in it, but it has the serious disadvantage of limiting the size of the organ or organism. The lobster and crab can continue to grow only by splitting the external skeleton. They shed this periodically, forming a new and larger one. Till this is formed they are weak and defenseless. If an erect plant were to split its external skeleton it would be too weak to stand. The limit which it sets to the size of the plant, rather than the difficulty of branching which is sometimes alleged as the disadvantage, is the serious defect in an external skeleton.

The grasses show an approach to an internal skeleton in that the greater part of the strength of the stem is due to the cylinders of supporting tissue in which the strands of conducting tissue are enclosed. But if the whole plant were to continue to grow, the cylinders in which the conducting tissues are enclosed would have to increase in diameter to allow an increase in the conducting tissues and this can not be done without splitting the strengthening cylinders and thereby greatly weakening the whole plant.

In the pines (using the word broadly) support and the conduction of liquids are accomplished by the same tissues, the same cells. These are the lowest plants in which an internal skeleton, if I may call it so, is found. Such a skeleton sets no limit to growth. It can be added to year by year as there is need of increased strength, and at the same time increased conducting tissue is formed. But conduction and mechanical support can not both be attained with the utmost efficiency and economy of material in cells which must serve both purposes. The diameter of the conducting elements must be limited lest they be weak, they must be comparatively short for the same reason, there can be no continuous tubes through which liquids can be rapidly transported. To ensure the requisite mechanical strength to the whole plant, the walls of the conducting cells must be thicker than would otherwise be necessary.

In the highest flowering plants, the dicotyledons, conducting and mechanically supporting tissues are combined in the same strands, but the same cells do not serve both purposes. In these plants, conducting and strengthening cells are side by side, they increase in number according to the needs of the plant, the conducting cells roost rapidly when most needed—as in the early spring—the strengthening cells later, when the increasing weight of the growing parts