CHAPTER I— THE MATERIAL INCOME OF PLANTS 



i. THE PLANT CELL 



An organ. — At a glance one sees that the body of an ordinary green 

 plant, such as a bean, is segmented, certain ] tarts being clearly marked 

 off by form from others. The colorless root grows in the soil; the green 

 shoot grows in the air and consists of a distinct stem with lateral out- 

 growths, the leaves. Anatomically, these parts 

 are members; but as the work of the plant is 

 distributed among them, each has its functions, 

 and physiologically each is an organ. 



A cell. — When one of the organs of the bean, 

 such as a leaf, is inspected, one sees that it, 

 too, is made up of parts, the petiole and the 

 leaflets. The latter are composed of ribs and 

 veins, with green tissue, or mesophyll, between. 

 These parts also have certain functions and 

 hence may be called organs. A microscopic 

 examination of the mesophyll reveals that it is 

 composed of minute bits of material which has 

 come to be known as living, and is called pro- 

 toplasm. Each, individualized, is a protoplast, 

 separated more or less completely from its neigh- 

 bors by membranes which it and they have 

 formed. The membrane and protoplast con- 

 stitute a eell (fig. 619). 



Organs of a cell. — When the protoplast is examined more closely, 

 a general translucent material, the cytoplasm, may be distinguished from 

 various inclusions. There are (a) many very minute particles, whose 

 nature is obscure, which tend to make the cytoplasm opaque; (b) minute 

 clear spaces, more fluid and sometimes watery, the vacuoles, many 

 of which coalesce as they enlarge with age, and form a few relatively 

 very large water spaces or only one; (r) a roundish nucleus; (J) numer- 



297 



Fig. 619. — Diagram of 

 mesophyll cell of a leaf; 

 chloroplast; n, nucleus; 

 vacuole ; w, cell wall. 



