24 



THE ELEMENTARY STRUCTDEE OP PLANTS. 



^□□□□□o 



We may often loc lirectly upon a delicate rootlet (as in Fig. 1), 

 or the petal of a flower, or a piece of thin and transparent sea-weed, 

 and observe the closed cavities, entirely circumscribed by nearly 

 transparent membranous walls. 



19. Does this cellular tissue consist of an originally homogeneous 

 mass, filled in some way with innumerable cavities ? Or is it com- 

 posed of an aggregation of little blad- 

 ders, or sacs, which by their accumu- 

 lation and mutual cohesion make up 

 the root or other organ? Several cir- 

 cumstances prove that the latter is the 

 correct view. 1. The partition between 

 two adjacent cells is often seen to be 

 double ; showing that each cavity is 

 bounded by its own special walls. 

 2. There are vacant spaces often to 



be seen between contiguous cells, where the walls do not entirely 

 fit together. These intercellular spaces are sometimes so large 

 and numerous, that many of the cells touch each other at a few 

 points only; as in the green pulp of leaves (Fig. 7). 3. When 

 a portion of any young and tender vegetable tissue, such as an 

 Asparagus shoot, is boiled, the elementary 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 coliere together, spontaneously separate as 

 the fruit ripens (Fig. 4, 5). 



20. The^ vegetable, then, is constructed of these cells or vesicles, 

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



or do not impress each other, they are generally 

 rounded or spherical. By mutual pressure they be- 

 come many-sided. In a mass of spheres each one is 

 touched by twelve others ; so, if equally impressed in 

 every direction, the yielding cells, flattening each 

 other at the points of contact, become twelve-sided ; and in a 

 section, whether transverse (as in Fig. 2) or longitudinal (as in 



PIG. 7. A magnifled 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 the spaces are large and copious in the rest of 

 the leaf, whore the cells are very loosely arranged, o, The epidermis or skin of the upper 

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



FIG. 8. View of a twelve-sided cell, detached entire, from tissue like that of Fig. 9. 



