32 PRACTICAL BOTANY. 



the mixture add a saturated solution of iodine 

 potassium iodide drop by drop, until precipitation 

 begins, This mixture stains the callus a deep 

 brown. 



e. Mucilaginous cell- walls. 



Resemble cellulose cell-walls in their reactions. On 

 treatment with iodine and sulphuric acid they some- 

 times assume a brownish colour in addition to the 

 blue. 



Cell- walls which have become converted into gum do 

 not turn blue on treatment with iodine and sulphuric 

 acid : Hanstein's aniline- violet colours them red. Both 

 mucilaginous and gummy cell-walls are stained by 

 inethylene blue. 



Mucilages stain pink with corallin solution ; certain 

 kinds stain with Hoffmann's blue. 



Gums stain with neither of these reagents. 



f. Mineral deposits in cell-walls. 

 i. Silica. 



On heating a section of tissue containing silica on 

 platinum foil with nitric acid, a complete skeleton of 

 the silicified cell-walls remains. 



(ii.) Calcium oxalate. 



Occurs in the form of crystals: insoluble in acetic 

 acid ; soluble, without evolution of gas, in nitric acid. 



(iii.) Calcium carbonate. 



Occurs either in distinct crystals, or, apparently, as 

 granules : soluble in acetic acid with evolution of 

 bubbles of gas 



The most characteristic form in which it appears is in special 

 outgrowths of the cell- wall which are incrusted with it ; these 

 are termed cystoliths (see p. 28). 



