THE RESERVE MATERIALS OF PLANTS. 



( Continued. ) 



IT may be considered unlikely that any of the dextrins 

 are to be regarded as true reserve materials. Where 

 they occur they are to be rather associated with the hydro- 

 lysis of starch and only very transitory, ultimately becoming 

 transformed into sugar, probably maltose. They may be 

 detected by giving a purple or reddish-brown colour with 

 iodine, as starch gives a blue one. It is not difficult to 

 detect dextrin in pollen tubes and to a less extent in the 

 germinating pollen grain from which the tube proceeds. 



Certain bodies giving a somewhat similar colour with 

 iodine have been noticed by Gris (33) and others in the 

 young cotyledons and the first leaves of Ricinus. While 

 resembling dextrin in this respect it is more likely that 

 these are starch grains in course of formation, the colour 

 agreeing with the observations of Eberdt, already referred 

 to. 



The study of the sugars as reserve materials is beset 

 with a good deal of difficulty. There is no doubt that a 

 very great variety of these bodies can be ascertained to 

 exist in various plants, and that a great many of them may 

 be looked upon as belonging to the circulating rather than 

 the reserve pabulum. Still there remain many deposits of 

 sugar which must be regarded as referable to the latter 

 category. The forms in which sugar most generally exists 

 in plants are no doubt glucose, maltose, lsevulose, and cane- 

 sugar. There are many others, but they exist in smaller 

 quantities and only in particular plants. Raffinose has 

 been shown by O'Sullivan (34) to be present in small 

 quantities in the ungerminated barley grain, and it can be 

 shown that it is available for the nutrition of the embryo, 

 though its nutritive value is on the whole small. 



Perhaps most interest centres in the role played by 

 cane-sugar ; it is best known as occurring in quantity in 



