CELLULAR TISSUE. 23 



of plants, moreover, is composed of three elements only, namely, 

 Carbon, Hydrogen, and Oxygen. The tissue of animals com- 

 prises a fourth element, Nitrogen. Plants, as a necessary result of 

 assimilating their inorganic food, decompose carbonic acid and 

 restore its oxygen to the atmosphere. Animals in respiration con- 

 tinually recompose carbonic acid, at the expense of the oxygen of 

 the atmosphere and the carbon of plants. These definitions will 

 be verified, extended, and illustrated in the progress of this work. 



SECT. II. OF THE CELLS AND CELLULAR TISSUE OF PLANTS IN 



GENERAL. 



17. The question recurs, What is the organized fabric or tissue 

 of plants, and how is vegetable growth effected ? The stem, 

 leaves, and fruit appear to ordinary inspection to be formed of 

 smaller parts, which are themselves capable of division into still 

 smaller portions. Of what are these composed ? 



18. Cellular Structure, To obtain an answer to this question, we 

 examine, by the aid of a microscope, thin slices or sections of any 

 of these parts, such, for example, as the young rootlet of a seed- 

 ling plant. A magnified view of such a rootlet, as in Fig. 1, pre- 

 sents on the cross-section the appearance of a network, the meshes 

 of which divide the whole space into more or less regular cavi- 

 ties. A part of the transverse slice more highly magnified (Fig. 2) 

 shows this structure with greater distinctness. A perpendicular 

 slice (Fig. 3) exhibits somewhat similar meshes, showing that the 

 cavities do not run lengthwise through the whole root without in- 

 terruption. In whatever direction the sections are made, the cav- 

 ities are seen to be equally circumscribed, although the outlines 

 may vary in shape. Hence, we arrive at the conclusion, that the 

 fabric, or tissue, consists of a multitude of separate cavities, with 



ness is not certainly made out. But it is becoming more and more apparent, 

 that the absolute distinctions between plants and animals are not to be drawn 

 from this class of characters. Dr. Lindley's definition, that "a plant is a cel- 

 lular body, possessing vitality, living by absorption through its outer surface, 

 and secreting starch" is so far good that it indirectly recognizes the essential 

 function of vegetation, starch being one of its organic products; yet it is only 

 one special form under which the nutritive matter created by the plant occurs, 

 and is not so universal as cellulose itself. It is much as if animals were char- 

 acterized by the faculty of secreting fat. 



