5l6 BOTANY OF CROP PLANTS 



breadth. Immature fibers or unripe fibers may show no 

 evidence of internal structure, but are smooth, straight, and 

 flat. "Kempy" fibers or "dead cotton" are such that are 

 normal in structure a portion of their length, and have the 

 appearance of immature and overripe fibers for another 

 portion. The quality of fiber depends largely upon the 

 number and regularity of twists, and upon its length and 

 fineness. The mature cotton fiber is almost pure cellulose. 



Cotton Fibers Distinguished from Other Common Textile Fibers. — There 

 are two chief ways of distinguishing textile fibers, by microscopical examina- 

 tion and by chemical reactions. The cotton fiber is a flat, ribbon-like band 

 twisted in a characteristic manner. The flax fiber is a straight, untwisted, 

 cylindrical fiber, with peculiar transverse markings at intervals along its 

 length. Hemp fibers resemble those of flax, but they may be distinguished 

 from the latter by the peculiar forked ends which are nearly always exhibited, 

 whereas flax fibers never show this character. All wool fibers possess char- 

 acteristic overlapping scales. The silk fiber is smooth, structureless, trans- 

 parent and quite regular in diameter. 



There are many ways of distinguishing the fibers by observing their reac- 

 tions to various chemicals. The following short key will illustrate a few of 

 their characteristic reactions. 

 Dissolves in caustic potash. 



An alkali solution of the fiber treated with lead acetate colors fiber 



black. Wool. 

 The above treatment does not color the fiber. Silk. 

 Does not dissolve in caustic potash. 



With iodine and sulphuric acid the fiber swells and becomes green, 



With iodine and sulphuric acid the fiber swells and becomes blue. 



Immerse fiber in concentrated sulphuric acid for two minutes, wash 

 in water, treat with dilute ammonia, dry — fiber forms a gelatinous 

 mass soluble in water, Cotton. 

 With above treatment, fiber is not altered, Linen. 

 Species. — Watt, in his great work, describes 42 distinct species and varie- 

 ties of Gossypium. A number of them are known only in the wild state. 

 Gossypium, as a genus, is indigenous to tropical regions. It is now grown 

 under cultivation to the 40° latitude on either side of the equator. 



Watt divides the wild and cultivated cotton plants of the world into five 

 "sections," as follows: 



