56 THK MECHANICAL FUNCTIONS. 



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 integuments, or skins of animals. 

 The simplest and apparently the most elementary 

 texture met with in vegetables is formed of exceed- 

 ingly minute vesicles, the coats of which consist of 

 transparent membranes of extreme tenuity.^ Fig. 3 



* Schleiden has made the curious discovery that every vegetable 

 cell is the result of the developement of a very minute body, or 

 nucleus, which he terms a cytoblust. Its shape is globular, or oval, 

 and sometimes flattened, or lenticular; it is apparently composed 

 of minute grains, the outlines of which are not very distinct; and it 

 generally contains a still more minute, but well defined, central par- 

 ticle, which he terms the nucleolus. The size of the nucleus varies 

 from the 10,COOth to the 500th of an inch. The cytoblast, or more 

 probably the nucleolus, is the first visible germ of the structure 

 which is to be formed; and the developement of which takes place 

 by the rising, from the surface of the cytoblast, of a minute trans- 

 parent vesicle, which on its first appearance may be compared to a 

 watch-glass set on a watch. This vesicle gradually enlarges, spread- 

 ing at first equally in all directions, and constituting a cell ; and 

 afterwards expanding more in particular parts, so as to acquire the 

 peculiar form which characterizes the texture to be fabricated : such 

 texture being always in the first instance composed of an aggrega- 

 tion of similar cells. In the progress of developement, these cells 

 undergo various subsequent modifications; first, by extension in 

 particular directions ; secondly, by partial or general adhesions of 

 their membranes ; and thirdly, by depositions of other materials, 

 either in the substance of their coats, or within their cavities, or in 

 the intervening spaces. These materials are frequently deposited in 

 spiral lines on the membranes of the vesicles ; thus constituting the 

 spiral cells, and spiral vessels of plants, which are presently to be 

 noticed. This spiral formation appears to result from the rotatory 

 movement of a fluid along the walls of the cells, between them and 

 the central mass of gelatine. 



During a great part of this process, the nucleus, with its nucle- 



