Ill 



COMMON MICRO-CHEMICAL REACTIONS. 



A FEW further practical exercises will now be given, involving 

 the use of common methods and reagents, and leading to a 

 fuller knowledge of the appearance and reactions of the parts 

 of the cell, and of some of the bodies commonly contained 

 in it. 



I. Cell-walls. 



A. Cellulose Walls 



Take some ordinary unbleached " cotton wool/' which con- 

 sists of unicellular hairs from the surface of the seed of the 

 cotton plant (Gossyptum). Moisten first with alcohol, and then 

 soak in water. 



a. Mount a small quantity in water, and examine first with a 

 low, and then with a high power : observe 



1. The long, filamentous, unicellular hairs, which compose 



the " cotton wool," coiled irregularly together. 



2. The rather thick, highly-refractive and colourless cell- 



wall. 



3. The remains of granular protoplasm, which may still be 



seen within. 



b. Soak a small quantity of the cotton for a few minutes in 

 iodine solution in a watch-glass, mount in iodine solution, and 

 note the cell-walls stained slightly yellow. 



c. Mount a small quantity of the cotton which has been 

 thoroughly soaked with iodine, in a single small drop of con- 

 centrate sulphuric acid diluted with an equal volume of water : 

 the greatest care is to be observed in the use of this reagent, 



