VEGETABLE ELEMENTS AND TISSUES. 19 



CHAPTER III. 



VEGETABLE ELEMENTS AND TISSUES. 



WE may now enter upon the consideration of the 

 microscopic structure of objects, beginning with those 

 which are derived from the vegetable kingdom, as 

 they are more easily procured and prepared for exa- 

 mination than those belonging to the animal king- 

 dom j moreover they are not so transparent, and 

 hence are more readily distinguished under the micro- 

 scope, which is of importance in the case of an un- 

 practised observer. 



Cells. The elements of which all plants consist 

 are cells. Cells, in their simplest condition, are 

 microscopic, rounded, colourless, closed sacs or vesi- 

 cles, resembling small bladders (Plate I. fig. 2), and 

 consist of a thin, transparent, colourless, vegetable 

 skin or membrane (a) called the cell-wall. The cells 

 are well seen in a little of the pulp of an apple (fig. 2) , 

 or in a section of almost any soft part of a plant. 

 A high power is usually required to show them dis- 

 tinctly, on account of their minute size. The outline 

 of the cells is seen to be double, one line indicating 

 the inner, the other the outer, surface of the cell-wall, 

 the space between the two lines corresponding to the 

 thickness of the cell-wall. 



In the pulp of the apple, the cells are loosely con- 

 nected, and so retain their rounded form; but in 

 most parts of plants, the cells become crowded and 

 squeezed together, from their ordinary or normal 

 expansion being limited in certain directions, so as 

 mutually to alter each other's shapes. The sides 

 then lose their originally rounded form and outline, 

 becoming more or less straight (PL I. figs. 1 & 4), 



