ORGANIC SUBSTANCES AS PLANT FOOD 203 



molecule, therefore, may be considered as consisting of a number of 

 glucose molecules, connected with each other through the removal 

 of water. Accordingly, the formation of starch from glucose may 

 be given by the following general formula: 



nC 6 Hi 2 O — (n - 1)H 2 -* (C 6 Hi O5) n -H 2 O. 



This formula shows that a starch particle contains 1 molecule of 

 H2O more than is expressed in the usual formula (CeHioOs),,. 

 The conversion of starch into sugar may be expressed: 



(C 6 Hi O5)„-H 2 O + (n - 1)H 2 -> nC H 12 O 6 . 



Detailed investigations have shown that in the hydrolysis of 

 starch the formation of glucose is preceded first by the formation 

 of polysaccharides soluble in water, the dextrines, and then by the 

 disaccharide maltose. Conversely, in the formation of the starch, 

 the glucose molecules first unite in pairs, giving maltose, and 

 then the maltose molecules are polymerized. The exact character 

 of these intermaltose unions is not known. More recent investi- 

 gations of Karrer and others make it seem highly probable that 

 starch is built up of ring compounds, anhydrides of sugars, in 

 which each ring, a diamylose, represents an anhydride of maltose. 

 These rings, linked to each other by means of side valences, are 

 thus polymerized into the starch molecule. The number of such 

 rings in a molecule of starch, however, is not as large as was 

 formerly supposed, probably only three or four. The enormous 

 molecular weight of starch obtained by cryoscopic determinations 

 is due to its heating in water, which gives colloidal solutions. This 

 results in the formation of rather large and very stable aggregates, 

 which upon the action of hydrolyzing agents fall apart, as a rule, 

 only at the time of the rupture of the valences, which hold together 

 the rings in the molecule. The actual molecular weight of starch 

 may be represented therefore by the formula (C6HioOs)6 or 

 (CtjHioC^s. 



Starch is not soluble in cold water, but it swells considerably in 

 it, absorbing as much as 40 per cent of water. This water is 

 retained with a great force. Even in an air-dry condition starch 

 will hold as much as 15 per cent water. Starch has a high specific 

 gravity (1.5 to 1.6). It precipitates easily and hence may be sep- 



