66 THE MECHANICAL FUNCTIONS. 



tion ; secondly, the Stems, which support them 

 in the proper position, or raise them to the re- 

 quisite 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 tegu- 

 ments, or skin of animals. 



The simplest and apparently the most ele- 

 mentary texture met with in vegetables is formed 

 of exceedingly minute vesicles, the coats of 

 which consist of transparent membranes of 

 extreme tenuity. Fig. 3 is a highly magni- 

 fied representation of the simplest form of these 

 vesicles.* But they generally adhere together 

 more closely, composing by their union a species 

 of vegetable cellular tissue, which may be re- 

 garded as the basis or essential component ma- 

 terial of every organ in the plant. This cellular 

 structure is represented in figures 4 and 5, as it 

 appears in the Fucus vesiculoms ; the first being 

 a horizontal, and the second a vertical section 

 of that plant. | The size of these cells differs 

 considerably in different instances. Kieser states 

 that the diameter of each individual cell varies 

 from the 330th to the 55th part of an inch ; so 



* These cells are well represented in the engravings which 

 illustrate Mr. Slack's memoir on the elementary tissue of plants, 

 contained in the 49th volume of the Transactions of the Society 

 of Arts. 



f De Candolle, Organographie Vegetale. 



