LINEN, COTTON, SILK, WOOL. 31 



Put on high power. The linen will be seen to consist of long, 

 cylindrical fibres, thickened at intervals into nodes, with a 

 small canal looking like a line running lengthwise of the fibre. 

 At intervals there are faint cross-lines (Fig. 26, B). The walls 

 are faintly striated and rather thick, and the canal, which is 

 uniform in width, may contain granular remains of protoplasm. 

 The faint cross-lines become more prominent when the fibres 

 are mounted in a concentrated aqueous solution of chloral 

 hydrate. 



Linen fibres are of vegetable origin, their material is cellulose, 

 a substance which is one of the chief materials found in plants. 

 Remove the cover glass, add a drop of a solution of iodine in 

 potassium iodide (see reagents), after a few minutes render 

 the fibres nearly dry by removing the liquid with filter paper, 

 then add a few drops of sulphuric acid (see reagents), replace 

 the cover glass and examine again. The fibres are stained a 

 deep blue color and swollen. This is a characteristic test for 

 cellulose material. Iodine alone does not color it, but the acid 

 acts on it, converting it into a starch-like body called amyloid, 

 which stains just like starch itself with iodine. Cellulose and 

 starch belong to the same group of chemical compounds, known 

 as carbohydrates. 



Mount some of the fibres in ammonio-copper hydroxide solu- 

 tion (see reagents) and note that they swell and dissolve 

 quickly, except a slender thread from the center (contents of 

 the canal) . The reagent is one of the few solvents for cellulose. 



Linen fibres are the so-called bast fibres found in the inner 

 bark of the stem of the Flax plant, Linum usitatissimum. 



COTTON. Mount a little raw cotton or non-absorbent cotton 

 wool in a drop of alcohol ; let most of the latter evaporate, then 

 add sufficient water and cover with a glass. With low power, 

 slender, clear fibres, not very different in appearance from linen 

 fibres, will be seen. Some of them are marked by what appears 

 to be constriction. With high power, long, flat bands, which 

 have caved in, often twisted like a corkscrew, at times striated 

 diagonally, will be seen (Fig. 26, C and D). The fibres do not 

 possess cross-lines. They are the long hairs on the seeds of the 

 cotton plant, Gossypium, the hairs being plant cells, consisting 

 at maturity only of cellulose walls which fall together, giving 

 the fibres the appearance of a flat band. The filaments are 

 about 2 cm. (*/. inch) long in short staple to 4 cm. (1 3 / 5 in^h) 

 long in long staple cotton, and about 0.02 mm. (0.0008 inch) 

 broad. There is a central canal running through each fibre, 

 much larger than in linen fibres. The fibres respond to tests 

 for cellulose as in case of linen. With ammonio-copper hy- 

 droxide solution, when the swelling action is moderated, there 

 often appear constrictions alternating with large swellings. 

 This is due to the cuticle which covers the surface of the fibres. 



