244 POPULAR SCIENCE MONTHLY. 



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, form- 

 ing 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 conduc- 

 tion 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 con- 

 duction and mechanical support can not ])oth 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 mechan- 

 ical 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, con- 

 ducting 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 strength- 

 ening cells later, when the increasing weight of the growing parts 



