224 HOW CROPS GROW. 
magnified 50 diameters. The cells mostly cohere together, 
but readily admit of separation. 
Structure of the Cell.-By the aid of the microscope 
it is possible to learn something with regard to the inter- 
nal structure of the cell itself. Fig. 29 exhibits the ap- 
pearance of acell from the flesh of the Jerusalem Arti- 
choke, magnified 230 diameters; externally the membrane, 
or wall of the cell, is seen in section. This membrane is 
filled and distended by a transparent 
liquid, the sap or free water of vegetation. 
“ Within the cell is observed a round body, 
b b, which is called the nucleus, and upon 
this is seen a smaller nucleolus, c. Lining 
the interior of the cell-membrane and 
connected with the nucleus, is a yellowish, 
turbid, semi-fluid substance of mucilagi- 
nous consistence, a, which is designated 
the protoplasm, or formative layer. This, when more 
highly magnified, is found to contain a vast number of 
excessively minute granules. 
By the aid of chemistry the microscopist is able to dis- 
sect these cells, which are hardly perceptible to the unas- 
sisted eye, and ascertain to a good degree how they are 
constituted. On moistening them with solution of iodine, 
and afterward with sulphuric acid, the outer membrane— 
the cell-~wall—shortly becomes of a fine-blue color. It is 
accordingly cellulose, the only vegetable substance yet 
known which is made blue by iodine after, and only after, 
the action of sulphuric acid. At the same time we observe 
that the interior, halfliquid, protoplasm, has coagulated 
and shrunk together,—has therefore separated from the 
cell-wall, and including with it the nucleus and the smaller 
granules, lies in the center of the cell like a collapsed 
bladder. It has also assumed a deep yellow or brown 
color. If we moisten one of these cells with nitric acid, 
the cell-wall is not affected, but the liquid penetrates it, 
