76 THE MECHANICAL FUNCTIONS. 



various descriptions, and in different stages of 

 transition. The density of the woody fibres 

 increases in proportion as these incrustations are 

 formed, till they have become nearly impervious ; 

 and have acquired a degree of rigidity peculiarly 

 fitting them for the office of giving mechanical 

 support to the fabric of the plant.* Their assem- 

 blage thus constitutes a kind of frame-work for 

 the whole system, which may be regarded as the 

 skeleton of the plant. Thus, what are called the 

 Jibres of leaves (Fig. 19), are principally com- 

 posed of these woody fibres, distributed in the 

 manner best adapted to support the expansion 

 of the soft and pulpy substance of those im- 

 portant organs. 



Besides the minute cavities of the cellular 

 tissue, there occur, in various parts of a plant, 

 much larger spaces, apparently serving the 

 purpose of reservoirs of particular fluids ; but 

 sometimes containing only air. Large air cells 

 are, in particular, met with very commonly in 

 aquatic plants, where they probably contribute 

 to impart the requisite degree of buoyancy. 



There are also contained, in the interior of 



* By drying different specimens of wood in a stove, Count 

 Rumford was led to the conclusion that the specific gravity of 

 the solid matter which constitutes timber is nearly the same in 

 all trees. He found that the woody part of oak, in full vegeta- 

 tion, constitutes only two-fifths of the whole bulk: and that 

 ordinary dry wood contains above one-fourth of its weight of 

 water. Thomson's Annals of Philosophy, I. 388. 



