VEGETABLE ORGANIZATION. ()7 



may be regarded as the skeleton of the j)laiit. 

 Tlius, what are called the Jihres of leaves (Fig. Hi), 

 are principally composed of these woody fibres, dis- 

 tributed 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, 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 pro- 

 bably contribute to impart the requisite degree of 

 buoyancy. 



There are also contained, in the interior of vege- 

 tables, certain organs, denominated Glands, which 

 are composed of closely compacted cells, and which 

 perform the function of secretion, that is, the con- 

 version of the nutritious juices into particular pro- 

 ducts required for various purposes in the economy 

 of the plant. 



The external parts of a living plant require pro- 

 tection against the injurious effects of the atmos- 

 phere, and of the moisture it deposits. 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 the 

 stem and branches, and interposing a barrier to 

 the action of fluids, or other extraneous bodies, on 

 the living organs. The cuticle is formed originally 

 by the condensation of a layer of cellular tissue, of 

 which the cells, being consolidated by exposure to 

 the air, and by compression, compose a thin but 

 impervious pellicle. Amici has distinctly shown, 



