THE CELL AND ITS STRUCTURE. 



17 



the characteristic test for starch, by turning it blue, 

 .ifFords a test for cellulose by turning it yellow ; but 

 if next there is added a drop of strong sulphuric acid, 

 then the cellulose turns blue. This reaction may be 

 watched under the microscope in a unicellular plant 

 or a bit of plant tissue, taking care to distinguish the 

 tint of the cell-wall from that of the starch, which is 

 coloured blue by the iodine alone. In vegetable cells 

 there are spaces or vacuoles filled with sap ; these are 

 often so large that the protoplasm looks like a mere 

 lining clinging to the cell- wall. Hence the protoplasm 

 of cells was often spoken of, in the early days of 

 biological observation, as the primordial utriclej-*^ a 

 name which may still be met with in some books. 



Cells, except in a few exceptional instances, are so 

 small that they cannot be clearly distinguished with- 



Fig. 5.— Typical cells of plants magnified to show protoplasm surrounded 

 by a cell -wail. A, Cells from a hair of a thistle-leaf ; B, cells from the tissue 

 of a small green leaf. In the former there is no chlorophyll, and the nucleus 

 is seen lyinor in the slightly granular protoplasm ; in the latter the nucleus is 

 not seen, being obscured by chlorophyll bodies ; v, large vacuoles filled with 

 ceU-sap; u, nucleus; w, empt.r cell-walls. (For typical animal cell, compare 

 and contrast Fig. 7, p. 22, and Tig. 10, p. 42.) 



' Utricle, i.a. a little bladder or bag (diminutive from Lat. 

 uter, a skin) ; primordial, i.e. belonging to the first construc- 

 tion (Lat. ordicn; to begin a piece of treaving). 



C 



