THE ASSIMILATING CELL. 17 



of the cell, whereby water is extracted from the cell-sap contained 

 in the central vacuole ; the substance causing this disturbance, 

 here acetic acid, is known as the plasmolyte. The reaction 

 shows that the protoplasm lines the cell-wall in the form of a 

 sac, which encloses the central vacuole ; concentrated solutions 

 of any salt (for instance, sodium chloride) act as plasmolytes, 

 the osmotic balance being so delicate that any but the most 

 dilute solutions will upset this balance causing plasmolysis. 

 Certain solutions of a definite strength and known as isotonic 

 Bolntioits do not cause plasmolysis (see section on Osmosis, 

 Chap. X.). 



(b) Iodine solution -added to a fresh preparation causes at first 

 a partial plasmolysis, which, however, does not obscure the 

 following important effects : — 



i. A darkening of some of the granules in the protoplasm ; these are 



stareh-gpanules fully formed. 



ii. A darkening of portions of the chloroplasts, this being due to the 

 eflfeot of the iodine upon granules of reserve Starch undergoing formation 

 in the substance of these bodies. 



iii. The nucleus and nucleolus are coloured brown (reaction forproteid). 



(c) Schuke's solution, added to a fresh preparation, acts first upon 

 the cell-wall, which turns blue ; the other effects noticed are 

 similar to those of (6), except that the starch-granules turn a 

 somewhat brilliant blue colour in contradistinction to the rather 

 deeper blue caused by iodine solution alone. 



(d) Iodine solution followed by a drop of pure sulphuric acid turns 

 the cell-wall blue. This reaction shows that the cell-wall, more 

 especially that of the young cell, is composed of cellulose ; the 

 first action of Schulze's solution shows the same thing. Pure 

 sulphuric acid alone will cause the protoplasm to assume at first 

 a pink colour (when sugar is present) owing to its action 

 upon the sugar, furfuraldehyde being produced. Cellulose is 

 dissolved by strong sulphuric acid with the formation of 

 dextrose. 



The presence of granules of starch in the interior of the 

 chloroplasts, a point brought out by reaction (b), indicates that 

 these bodies are active starch manufacturers and storers. In 

 Vallisneria and Begonia leaves all stages in the production of starch- 

 granules may be traced in the chloroplasts, from the minutest 

 particle shown up by the iodine solution, to the fully-formed 



