60 THE MECHANICAL FUNCTIONS. 



according to the respective functions which they are called 

 upon to perform. 



We shall now examine the several kinds of texture in re- 

 lation to these functions, in the order of their increasing 

 complexity; heginning with those of vegetables, which are 

 apparently the simplest of all. 



§ 2. Vegetable Organization, 



Plants, being limited in their economy to the functions 

 of nutrition and reproduction, and being fixed to the same 

 spot, and therefore in a comparatively passive condition, re- 

 quire for the performance of these functions mechanical con- 

 structions of a very different kind from those which are ne- 

 cessary to the sentient, the active, and the locomotiv^ani- 

 mal. The organs that are essential to vegetables are those 

 which receive and elaborate the nutritive fluids they require, 

 those which are subservient to reproduction, and also those 

 composing the general frame-work, which must be super- 

 added to the whole for the purpose of giving mechanical 

 support and protection to these finer organizations. As 

 plants are destined to be permanently attached to the soil, 

 and yet require the action both of air and of light; and, as 

 they must also be defended from the injurious action of the 

 elements, so we find these several objects provided for by 

 three descriptions of parts: namely, first, the Roots, which 

 fix plants in their situation; secondly, the Stems, which 

 support them in the proper position, or raise them to the 

 requisite height above the ground; together with the 

 branches which are merely subdivisions of the stem; and 

 thirdly, the external coverings, which correspond in their 

 office to the teguments, or skin of animals. 



The simplest and apparently the most elementary texture 

 met with in vegetables is formed of exceedingly minute ve- 

 sicles, the coats of which consist of transparent membranes 

 of extreme tenuity. Fig. 3 is a highly magnified represen- 

 tation of the simplest form of these vesicles.* But they ge- 



* These cells are well represented in the engravings which illustrate Mr. 



