43 



HOW CKOPS GROW. 



Uearly all plants and-a lL ^^pi't j^aris f^nt°i" pollnlnao, 

 but it is relabively moat, abundant in 

 Sififfl^ and leaves. In' RPRds ^'''i fo-^"^= "- 



outer -Coating, but in the intmot-of-tlie 

 seed-it-ex-ifits in smflfll^pi^eftion. 



The fibers of cotton (Fig. 11, a), hemp, 

 and flax (Fig. 11, b), and white cloth and 

 unsized paper made from these materials, 

 are nearly pure cellulose. 



The fibers of cotton, hemp, and flax are simply 

 long and thick-walled cells, the appearance of 

 which, when highly magnified. Is shown in Fig, 11, 

 where a represents the tliinner, more soft, and col- 

 lapsed cotton fiber, and 6 the thicker and more dur- 

 able fiber of linen. 



Wood, or woody fiber, consists of long 

 and slender cells of various forms and di- 

 mensions (see p. 393), which are delicate 

 when young (in the sap wood), but as 

 they become older fill up interiorly by the deposition of re- 

 peated layers of cellulose, which is more or less inter- 

 gro(vn with other substances.* The hard shells of nuts 

 and stone fruits contain a basis of cellulose, which is im- 

 pregnated with other matters. 



When quite pure, cellulose is a white, often silky or 

 spongy, and translucent body, its appearance varying 



* Wood was formerly supposed to consist of cellulose and so-callfed 

 " lignln." On this view, according to F. Sohulze, lignin Impregnates 

 (not simply incrusts) the cell-Avall, is soluble in hot alkaline solutions, 

 and is readily oxidized by nitric acid. Schulze ascribes to it the com- 

 position 



Carbon ., 55.3 



Hydrogen .' 5.8 



Oxygen 38.9 



Fig. 11. 



This is, however, simply the inferred composition of what is left after 

 the cellulose, etc., have been removed. "Lignin" cannot be separated 

 in the pure state, and has never been analyzed. What is thus desig- 

 nated is a mixture of several distinct substances. Fremy's lignose, lig- 

 none, lignin, and lignireose, as well as J. Erdman's glycolignose and 

 lignose, are not established as chemically distinct substances. 



