VEGETABLE CELLS 31 



cording to circumstances, may be deposited, e. g. fat, 

 starch, pigment, albumen (cell-contents). But also, quite 

 independently of these local varieties in the contents, we 

 are enabled, by means of chemical investigation, to 

 detect the presence of several different substances in the 

 essential constituents of the cells. 



The substance which forms the external membrane, 

 and is known under the name 

 of cellulose, is generally found 

 to be destitute of nitrogen, and 

 yields, on the addition of iodine 

 and sulphuric acid, a peculiar, 

 very characteristic, beautiful 

 blue tint. Iodine alone produces no colour ; sulphuric 

 acid by itself chars. The contents of simple cells, on the 

 other hand, do not turn blue ; when the cell is quite a 

 simple one, there appears, on the contrary, after the addi- 

 tion of iodine and sulphuric acid, a brownish or yellowish 

 mass, isolated in the interior of the cell-cavity as a spe- 

 cial body (protoplasma), around which can be recognised 

 a special, plicated, frequently shrivelled membrane (pri- 

 mordial utricle) (fig. 1, c). Even rough chemical analysis 

 generally detects in the simplest cells, in addition to 

 the non-nitrogenized (external) substance, a nitrogenized 

 internal mass ; and vegetable physiology seems, there- 

 fore, to have been justified in concluding, that what really 

 constitutes a cell is the presence within a non-nitro- 



Fig. 1. Vegetable cells from the centre of the young shoot of a tuber of 

 Solanum. tuberosum. a. The ordinary appearance of the regularly polygonal, 

 thick-walled cellular tissue, b. An isolated cell with finely granular-looking cavity, 

 in which a nucleus with nucleolus is to be seen. c. The same cell after the addition 

 of water; the contents (protoplasma) have receded from the wall (membrane, 

 capsule). Investing them a peculiar, delicate membrane (primordial utricle) 

 has become visible, d. The same cell after a more lengthened exposure to the 

 action of water ; the interior cell (protoplasma with the primordial utricle and 

 nucleus) has become quite contracted, and remains attached to the cell-wall (cap- 

 sule) merely by the means of fine, some of them branching, threads. 



