294 PHYSIOLOGY OF NUTRITION. 



rhizome, we select for examination the root-stock of Pteris aqui- 

 lina, which creeps horizontally in the soil. We may use alcohol 

 material, and select not too thick pieces of the rhizome. The 

 ground tissue consists chiefly of parenchyma, the cells of which 

 contain very large quantities of starch, and is traversed by very 

 strongly developed plates of sclerenchyma, observable even in 

 macroscopical examination of transverse sections of the rhizome 

 as broad, dark lines. Between these plates of sclerenchyma the 

 bfcollateral vascular bundles are readily recognised. Each vas- 

 cular bundle is surrounded by a single layer of cells rich in starch 

 (phloem sheath, Vorscheide) and by the endodermis proper, which 

 is however free from starch. 



We shall often recur to the significance of starch stored up in 

 receptacles of] reserve material. It need only be mentioned here 

 that starch is the most important non-nitrogenous reserve material 

 in plants, and that when it is to be employed in the development 

 of this or that organ it must first be converted into soluble com- 

 pounds which can leave the storehouses. Under the influence of 

 diastatic ferments, viz., starch is converted, as we shall see in 112, 

 into glucose, and thus its translocation is rendered possible. 



1 See Sanio, Ihitersucliunyen iibcr die im Winter Stcirke filhrendcn Zellen de* 

 Holzes, Halle, 1858. 



111. The Quantitative Determination of Starch. 



Much has already been said elsewhere as to the properties and 

 behaviour of starch. At this place the chief object is to indicate 

 the method to be employed in quantitative determinations of 

 starch. Starch as such is, as is known, unable to reduce Fehling's 

 solution. It can, however, be converted into grape-sugar by the 

 action of acids, and this is readily estimated quantitatively by 

 means of Fehling's solution. 2-3 gr. of pure potato starch, which 

 has been freed from water at a temperature of 100-110 C., is 

 heated in a flask with 200 c.c. of water. To the fluid is added 20 

 c.c. of 25 per cent. Hydrochloric acid, and it is then heated for three 

 hours in an actively boiling water-bath, the water lost by evapo- 

 ration being replaced.* After cooling, the fluid is neutralised 



* Sulphuric acid has also been used for converting starch into grape-sugar. 

 Hydrochloric acid, however, is to be preferred. SeeE. Sachsse, Phytochemisclic 

 Unters., 1880, p. 47. 



