VEGETABLE ORGANIZATION. 67 



tlieir development, large additions of solid matter. These 

 fibres are generally collected together into bundles or layers, 

 and are accompanied by cells and vessels of various descrip- 

 tions, and in different stages of transition. The density of 

 the woody fibres increases in proportion as these incrusta- 

 tions 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 assemblage 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 fibres 

 of leaves (Fig. 19,) are principally composed of these woody 

 fibres, distributed in the manner best adapted to support the 

 expansion of the soft and pulpy substance of those important 

 organs. 



Besides the minute cavities of the cellular tissue, there 

 occur, in various parts of a plant, much larger spaces, appa- 

 rently 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 vegetables, 

 certain organs, denominated Glands, which are composed 

 of closely compacted cells, and which perform the function 

 of secretion, that is, the conversion of the nutritious juices 

 into particular products required for various purposes in the 

 economy of the plant. 



The external parts of a living plant require protection 

 against the injurious effects of the atmosphere, and of the 

 moisture it deposites. For this purpose there is provided a 

 membrane, termed the Cuticle, which is spread over the 

 whole surface, investing the leaves and flowers, as well as 



* By dry'mg diflTerent specimens of wood in a stove, Count Rumford was led 

 to the conclusion that the specific gravity of the solid matter which consti- 

 tutes timber is nearly the same in all trees. He found that tlie woody part 

 of oak, in full vegetation, 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. 



