UTRICULAR STRUCTURES, 



187 



the membrane of both begins to be thickened soon after 

 its production. They are distinguished by the fact that in 

 the starch-utricles this thickening is relatively much more 

 considerable, even at the very beginning, than in the cell. 



The structure of the starch-utricles first becomes clear 

 when they have acquired a greater size ; this is : a mem- 

 brane, probably composed of gelatine, layers formed of 

 starch, and a cavity filled with watery fluid. The mem- 

 brane is more or less distinct ; when it is thick enough 

 we may perceive that it remains uncoloured in the treat- 

 ment with iodine, while the layers become blue or violet 

 (%. 14 15). 



The layers are formed like the thickening layers of the 

 cell ; the concentric lines are to be explained in the same 

 way as these. The layers are composed of starch through- 

 out their entire thickness. They decrease in solidity 

 from without inward, and contain more water in the 

 reversed order. Hot water and acids cause them to swell 

 up very much, and they frequently tear in consequence 

 of an unequal expansion, either merely in the interior, or 

 from within outward. But that an air-bubble originates 

 in the cavity through the action of sulphuric acid, as has 

 been asserted, I have not seen, neither can I conceive how 

 sulphuric acid could produce an air-bubble here, by ab- 

 straction of water. For the acid does not draw out the 

 water like some pump or exhausting apparatus, bat when 

 the acid is applied to the starch-grain a reciprocation of 

 currents is set up, according to the law of endosmosis 

 and exosmosis, through the layers and the membrane, the 

 acid passing in, and a portion of the fluid of the cavity 

 and the layers going out. If more fluid passed out than 

 in, and the starch-utricle could not contract in a corre- 

 sponding degree, the only consequence would be that the 

 diminution of density, beginning in the interior, would 

 cause an increased flow inwards from without. Now the 

 swelling up of the utricle proves exactly the contrary, 

 namely, that more fluid passes in than out. 



The cavity of the starch-utricle is sometimes larger, 

 sometimes smaller. In a given size of the utricle of 



