244 II. von Mohl on the supposed Eocistence 



as is evident from the circumstance that all the grains of Canna 

 from which the soluble substance is entirely removed are tra- 

 versed by cracks^ some tolerably wide, which run out like rays 

 from the centre to the periphery, and are wider in the middle 

 than in the centre and at the periphery, towards which latter 

 they generally thin off; — but when they reach the surface, they 

 frequently cause the grains to fall into sharp-cornered triangular 

 fragments. As these cracks are not connected with the forma- 

 tion of a cavity in the centre of the grain, they do not originate 

 by the inner laminse becoming contracted in the radial direc- 

 tion with a simultaneous drawing outwards towards the firmer 

 outer layers, causing a lateral extension until they are torn 

 across, but the cracks evidently arise from the contraction of 

 the laminse of the grains being stronger in the direction of the 

 tangent than in the radial direction. 



The lamination undergoes no change by the treatment with 

 saliva. In many cases, particularly when a slow solution of 

 the soluble substance of the grain takes place in the living plant 

 (for example, in germinating wheat), the lamination becomes 

 much more evident in the earlier stages of the process than it 

 was previously, as the solution does not affect all the layers uni- 

 formly, but at the outset shows itself only in the looser laminse, 

 and thereby considerably increases the already existing difference 

 of density of alternating laminse. But when the soluble portion 

 has been entirely extracted from all the laminse, and the whole 

 grain has thus acquired far greater transparency, the lamination 

 is mostly far more difficult to perceive than in the fresh grain, 

 as may be seen particularly in the very distinctly laminated 

 grains of the potato. 



The substance of the grains which have been exhausted by 

 saliva, so long as they are saturated with water, is in a high 

 degree brittle, and a slight pressure on the covering-glass causes 

 it to crumble into sharp-cornered fragments. When dried, it 

 contracts considerably, which renders the lamination much more 

 indistinct ; and a re-wetting with water does not always restore 

 it in the original clearness. 



Grains with the starch extracted act upon polarized light 

 exactly like unaltered starch-grains ; i. e., when the direction of 

 their laminse is observed, in the opposite manner to cellulose 

 membranes. In a former paper in this Journal*, on the inves- 

 tigation of vegetable tissues by the aid of polarized light, I re- 

 garded this diverse action as the result of a chemical constitu- 

 tion of the starch-grain differing from that of cellulose. The 

 idea might certainly have been entertained that it depended 

 upon conditions of tension in the separate layers of the starch- 

 * Annals, 3rd series, vol. i. pp. 198, 263. 



