CELLULAR TISSUE. 25 



a portion of any young and 

 tender vegetable tissue, such as an As- 

 paragus shoot, is boiled, the elementa- 

 ry cells separate, or may readily be 

 separated by the aid of fine needles, 

 and examined by the microscope. 4. In 

 pulpy fruits, as in the Apple, the walls 

 Of the cells, which at first cohere to- 

 gether, spontaneously separate as the 

 fruit ripens (Fig. 4, 5). 



20. The vegetable, then, is constructed of these cells or vesi- 

 cles, much as a wall is built up of bricks. When the cells are 

 separate, or do not impress each other, they are generally round- 

 ed or spherical. By mutual compression they become polyhedral. 

 As in a mass of spheres each one is touched by twelve others, if 

 equally impressed in every direction, the yielding cells become 

 twelve-sided ; and in a section, whether transverse (as in Fig. 2) 

 or longitudinal (as in Fig. 3), the meshes consequently appear six- 

 sided. If the organ is growing in one direction more than another, 

 the cells commonly lengthen more or less in that direction, and 

 thus become oblong, cylindrical, or tubular when nearly free, or 

 prismatic when laterally impressed. If the force of extension, 

 compression, or nutrition be greater in one direction than another, 

 or unequal on corresponding sides, a corresponding variety of 

 form is produced. It is not necessary to detach a cell in order 

 to ascertain its shape ; that may usually be inferred from the out- 

 lines of their section in two or three directions. Nor have the 

 forms precise geometrical regularity ; they merely approach more 

 or less closely the figures to which they are likened. 



21. The walls of the cells are transparent, at least in their early 

 state, and almost always colorless. In a few cases the membrane 

 itself is said to have a tinge of green, and in the stems of Ferns it 

 is often brown. The various colors which the parts of the plant 

 present, the green of the foliage, or the vivid hues of the corolla, 



FIG. 7. A magnified section through the thickness of a leaf of Illicium Floridanum, show- 

 ing the irregular spaces or passages between the cells, which are small in the upper layer of 

 the green pulp, the cells of which (placed vertically) are well compacted, so as to leave only 

 minute vacuities at their rounded ends ; but they are large and copious in the rest of the leaf, 

 where the cells are very loosely arranged, a, The epidermis or skin of the upper, b, of the 

 lower surface of the leaf, composed of perfectly combined thick-walled cells. 



3 



