STARCH-GRAINS. 



337 



thousandths or hundredths of a mm., and an average specific weight of i-^6. In 

 consequence of these properties, and since they are insoluble in cold water, it is easy 

 to obtain the starch-grains in quantity from starchy tubers, seeds, and other organs : 

 it suffices to rub or otherwise crush such parts of the plant, just as ordinary wheaten 

 flour is obtained by grinding grains of wheat, and then to remove the crushed masses 

 of tissue with abundance of water, to' obtain pure starch-meal as a sediment. On 

 observation with the microscope the somewhat larger starch-grains in the cells are 

 conspicuous by their strong refractive power and corresponding lustre. On the addi- 

 tion of solutions of iodine they become coloured, according to the quantity of iodine 

 in each case, of various shades from bright blue to blue-black. The structure of 

 the starch-grain has been investigated very 

 thoroughly, especially by Naegeli (1859), from 

 whose researches a deeper insight into the 

 innermost structure of organised bodies was 

 first obtained. It is not necessary, however, 

 to go further into these matters here. In the 

 meantime it suffices to bring forward the fact 

 that starch-grains may possess the most various 

 shapes, and indeed that every plant produces 

 them of characteristic form. These character- 

 istic forms, however, only*appear in the large, 

 completely developed grains ; granules which 

 are still young and very small being mostly 

 spherical, or having the form of segments 

 of spheres, and only developing the charac- 

 teristic shapes on subsequent growth. 



To illustrate a few of these only, I may 

 refer in the first place to Fig. 234, which 

 shows various forms of starch from the tubers 

 of the Potato. The large and simple grains 

 are here approximately ovoid, with an eccen- 

 tric hilum. The starch-grains of the Bean, 

 Pea, and other Leguminosese, on the other 

 hand, are ellipsoidal, with a fissure running 

 in the middle of the substance of the grain 

 in the dry state. The starch-grains of the 



Rye and Wlieat have the form of bi-convex lenses (Fig. 235 5); those of the 

 tubers of Ctoruma are elliptical tablets, which possess a sort of peg or handle at 

 one end, in which the so-called hilum lies. Where the starch-grains are aggre- 

 gated in large numbers and predominate in the cells of dry seeds, as in the 

 Maize and Rice, they may be polyhedral, and so densely crowded that only very 

 thin spaces filled with proteinaceous substance are left between them (Fig. 235 0). 

 These are all simple grains. 



Compound grains consist of several or many, or even thousands of individual 

 starch-grains, which, however, are so closely connected that they form together a 

 roundish grain, which with sufficient pressure breaks up into the individual granules ; 

 [3] 



Fig. 234.— starch-grains from the tuber of a Potato 

 (800). ^ an older simple grain ; B a semi-compound grain ; 

 C, D compound grains. E an older grain the hilum of 

 which is divided ; a a very young grain, i an older one 

 c one still older and with divided hilum. 



