32 



VEGETABLE HISTOLOGY. 



Linen has no cuticle and does not give the appearance men- 

 tioned. The cuticle is insoluble in the reagent, not being cellu- 

 lose in nature. 



WOOL. Mount some fibres from white woolen yarn in water. 

 With low power the fibres are clear, slightly roughened on the 

 surfaces with faint cross-lines. Under high power the fibres 

 are cylindrical, containing a central axial substance, called the 

 medulla (not present in all hairs or wool). The surface is cov- 

 ered by imbricated scales, like tiles on a roof, giving to the edges 

 of the fibres a barbed appearance (Fig. 26, E). Compare a 

 human hair with wool. If raw wool be examined, globules of 

 fatty matter (wool fat) will be seen adhering to the fibres. 



Pig. 26. 



Wool being of animal origin does not give the cellulose reac- 

 tions, as do linen and cotton fibres, but shows proteid reactions. 

 Treated with iodine solution and sulphuric acid, as described 

 under linen, the fibres are stained a deep yellow and do not dis- 

 solve even when warmed. W T hen warmed on the slide with a 

 saturated aqueous solution of picric acid the fibres are stained 

 yellow; cellulose does not stain with this reagent. Warmed 

 with ammonio-copper hydroxide solution, wool turns bluish- 

 violet and its structure becomes more distinct, but it does not 

 dissolve or swell. 



Wool varies in its details of structure, and the identification 



