28 THE VEGETABLE CELL IN GENERAL. 



and where there is any break in the continuity of the mass, for 

 instance in the case of sap-cavities, a similar limiting film may 

 be supposed to exist. 



122. The consistence of protoplasm depends on the amount 

 of water which it contains. Thus in drj- seeds it is nearly' as 

 tough as horn, while in the same seeds during germination it 

 becomes like softened gelatin. It absorbs water readilj- and be- 

 comes permeated by it, thereby increasing its apparent fluidit}-, 

 but it never becomes a true fluid. Moreover, there is a limit 

 to the amount of water which it takes up. 



123. Chemically considered, protoplasm is a very complex 

 substance. It belongs to a group of bodies of which the albumin 

 of egg may be conveniently taken as the type. They undergo 

 many slight but sometimes remarkable changes, and have been 

 collectively termed proteids. The terms albuminoids and pro- 

 teids may be used interchangeably (see 857). 



124. The albuminoids, or proteids, which form with water the 

 bulk of protoplasm proper, are of course associated with the 

 matters which this living substance makes, uses, and discards. 

 But these matters exist in the protoplasm in ver3- different pro- 

 portions at different times, though never in such amount as to 

 obscure the peculiar reactions of the albuminoids. These are 

 the following: 1. The yellow or brownish color imparted by 

 solutions of iodine. 2. The purple color produced when the 

 specimen first saturated with a solution of cupric sulphate is 

 acted on by potassic h3drate. 3. The rose color, often faint, 

 which follows the successive action of a solution of sugar 

 and strong sulphuric acid. 4. The red color given by Millon's 

 reagent. This test generallj' requires the cautious application of 

 heat. 5. The yellow or orange color following the application, 

 in succession, of strong nitric acid and amnionic hydrate. 



125. Dilute solutions of the caustic alkalies dissolve proto- 

 plasm ; concentrated solutions do not. If a young cell is acted 

 on b^' concentrated potash, its protoplasm is not essentially 

 affected ; but if water is now added, the protoplasm dissolves 

 at once. 



126. The spherical or ellipsoidal mass found in the protoplasm 

 of active cells, and difiering from the rest of the protoplasm in 

 its greater density, is the nucleus. The sharply defined point 

 often seen in the nucleus is the nucleolus. 



127. The nucleus undergoes remarkable changes during the 

 earliest stages of the cell, which will be described in the chapter 

 on "Growth." The relations which exist between the proto- 



