54 



THE ELEMENTARY STRUCTURE OF PLANTS. 



69 



they germinate ; and is the appropriate prepared material for the 

 plant's nourishment and growth. Dextrine is a substance inter- 

 mediate in nature between sugar and starch. 



81. Starch {Farina, Feculd) is one of the most important and 

 universal of the contents of cells, in which it is often accumulated 

 m great quantity, so as to fill them completely (Fig. 70) ; as in 



farinaceous roots, 

 seeds, &c. It oc- 

 curs in the pa- 

 renchyma of al- 

 most every part 

 of the plant, ex- 

 cepting the epi- 

 dermis : but while chlorophyll is nearly restricted to the superficial 

 parts, directly exposed to the light, starch is most abundant in inter- 

 nal or subterranean jiarts, concealed from the light, as in roots and 

 tubers, the pith of stems, and seeds. Starch consists of transparent 

 oval or rounded grains, sometimes becoming angular by mutual 

 pressure, as in rice. The size of the grains varies extremely in 

 different plants, and even in the same cell ; as in the potato, where 

 the larger grains measure from ■^\-^ to -^^^ of an inch in their larger 

 diameter, but the smallest only ^j'g^ of an inch. In wheat-flour the 

 larger grains are -g^i^ to -g-J^ of an inch in diameter. And the 

 largest starch-grains knowTi are -^^^^ of an inch long. Indeed, from 

 their formation, we might expect that their bulk would vary con- 

 siderably. The mode of their formation is indicated by the peculiar 

 markings, by which most starch-grains may be recognized ; namely, 

 by the dot or darker point which is seen commonly near one end of 

 the grain, and the fine concentric lines drawn around it. These are 

 best seen in starch from the potato, one of the most characteristic 

 forms and easiest to be examined, under a magnifying jiower of 

 from 250 to 500 diameters (Fig, 69). The chemical composition 

 of starch is exactly the same as that of cellulose (27) ; and the 

 grains are solid throughout, but their interior usually softer or more 

 gelatinous. The lines evidently show that starch-grains consist of 

 concentric layers, of diiferent density, successively deposited on an 



FIG. 69. Two cells of a potato, with some contained starch-grains, highly magnified ; one 

 of the cells contains a few cubical crystals also. 



FIG 70. A minute portion of Indian meal, strongly magnified ; the cells absolutely filled 

 with grains of starch. 



