36 PRACTICAL BOTANY 



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 concentrate sulphuric acid diluted with an 

 equal volume of water (the greatest care is to be 

 observed in the use of this reagent, so that it shall not 

 gain access to the stage, or the objective : only a very 

 small quantity is to be used, and the slide should l)e washed 

 in water directly the observation has been made). A low 

 power will suffice to show that 



1. The cell-walls swell greatly, and in an irregular 



form, and ultimately lose their sharp contour. 



2. They assume a blue colour. This colouring is 



often not uniform, and this reaction, though 

 trustworthy as positive evidence of the presence 

 of cellulose where the blue colour is obtained, 

 is not secure as proving the absence of cellu- 

 lose if the blue colour is not seen. 



Cell- walls, and certain of the cell-contents (protoplasm, starch- 

 grains, aleurone-grains, crystalloids) usually contain in their sub- 

 stance a certain amount of water termed the water of imbibi- 

 tion. The amount of this water of imbibition may be made to. 

 vary by appropriate reagents, and this involves variation in size 

 of the body observed : if the quantity of water of imbibition be 

 increased (for instance by the action of sulphuric acid as above 

 directed) the body swells, losing at the same time its high refrac- 

 tive power, as in the case of the cellulose wall when treated as 

 above with dilute sulphuric acid. Similar phenomena will be 

 described below in starch -grains, &c. Other reagents may with- 

 draw water, in which case a diminution of bulk will be the result. 



d. Mount a fresh piece of the soaked cotton in chlor- 



