SUGAR 129 



Cut up -very young Potatoes into small pieces, and treat 

 with picric acid : wash out carefully with dilute alcohol, and 

 harden. 



Cut sections parallel to the surface, and not far below it (since 

 the leukoplasts are best seen in the most superficial cells 

 immediately within the corky rind): treat the sections for a 

 few minutes in alcohol with a few drops of iodine solution 

 added ; or in very slightly diluted tincture of iodine, and mount 

 in pure glycerine : examine under a high power, and observe 



1. The cells of the usual parenchymatous type, with proto- 

 plasm and nucleus. 



2. Numerous spherical bodies, usually aggregated round the 

 nucleus : in some cells these will stain blue (starch), with a 

 small yellowish body attached which is the leukoplast ; in 

 other cells, which were nearer the outer surface of the tuber, 

 the bodies will stain uniformly yellowish brown : these are 

 the young leukoplasts which have not yet formed starch. 



II. Cellulose occurs as a reserve in the endosperm of the 

 Date, and other Palms : sections should be cut and the reactions 

 noted. Thickened cell-walls will also be found in sections of the 

 cotyledons of Lupinus. 



III. Sugars. 



(a) Grape-sugar. 



1. Cut a transverse section of a ripe grape, of such thickness 

 that some cells at least shall be uninjured : mount in water, 

 and observe under a low power the transparent parenchymatous 

 pulp, consisting of cells with thin walls, very sparing contents, 

 and large central vacuole. 



Treat for a few minutes with a relatively large bulk of 

 alcohol in a watch-glass : on re-examining, numerous crystals 

 will now be seen in the cells. 



Irrigate thoroughly with water : the crystals may be seen to 

 be re-dissolved : they consist of grape-sugar, which is in solu- 

 tion in the cell-sap of the living cell. 



2. Squeeze out the juice of some grapes into a test-tube : 

 add a little of Fehling's solution (see Appendix A), and boil : 

 a bulky yellow precipitate is formed owing to reduction of 

 the copper. 



K 



