258 Plant Physiology 



sago, tapioca, bread-fruit, banana, and many other vege- 

 tables and fruits. Reckoned in per cent of dry weight, 

 the potato tuber contains a starch content of about 80 

 per cent, while corn and many cereals may contain 60 

 per cent or more. According to Konig starch contains on 

 the average about 15 per cent water, 1 per cent nitrogenous 

 bodies, and generally much less than 1 per cent of ash. 



Upon hydrolysis starch yields, as subsequently shown, 

 first dextrins, then maltose, and this is ultimately trans- 

 formed to glucose, although some of the dextrin, (about 

 one fifth) is more resistant to hydrolysis. 



Inulin, an amylase less complex than starch, is char- 

 acteristic of the tuberous roots of Dahlia and of some other 

 composites, although occurring also in other plants. It 

 is dissolved in the cell-sap, but may be crystallized out as 

 spherites. These crystals are soluble in hot water. It 

 yields fructose on hydrolysis. 



145. Cellulose. The . cell-walls consist in large part 

 of a substance which passes under the general name of 

 cellulose w(C 6 Hj O 5 ). Cell-walls are frequently impreg- 

 nated with gummy, metallic, or other substances; this 

 is the usual case with epidermis, cork, wood, and the like. 

 Nevertheless, some form of cellulose forms a large per cent 

 of the walls of flowering plants. 



The celluloses proper resist hydrolysis with weak acids, 

 and except at the time the cell-walls are being laid down 

 they are unimportant in metabolism. Hemicelluloses are 

 forms which are readily hydrolyzed, yielding monosac- 

 charids other than glucose. They constitute, in fact, 

 the reserve cellulose deposited upon the cell-walls in the 

 endosperm of many seed and some other storage organs, 



