224 HOW CROPS GEOW. 



magnified 50 diameters. The cells iQostly 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 a cell 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, 

 -T> 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 mucUagi- 

 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, half-liquid, protoplasm,, has coagulated 

 and shrunk together, — lias 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 aflfected, but the liquid penetrates it, 



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