14 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



stage condenser, should the microscope be constructed to carry 

 one. 



Sometimes after focussing the preparation, or after moving 

 it, the figure may be seen to have lost its clearness. In all 

 probability this is because some of the mounting fluid from the 

 preparation has got upon the lower lens of the objective. This will 

 very readily occur if too large a quantity of fluid has been used, 

 and some has run out from under the edge of the cover-glass ; or 

 if pressure, e.g., of the objective upon the cover-glass, has forced 

 some out. Should this be the case, we must withdraw the tube 

 from its sheath, or else remove the objective, or turn the nose- 

 piece round ; and, if our supposition is correct, wipe the front lens 

 of the objective with a clean and often-washed piece of linen rag, 

 or Japanese rice paper, or, better still, rub it with a freshly broken 

 surface of a piece of elder pith. 



The starch grains of the potato-tuber attain a comparatively 

 large size, and show comparatively clear lamination. The laminae 

 are recognisable because, by reason of their varying density, they 



refract the light to differing 

 degrees. They are excen- 

 trically constructed, as their 

 organic centre, or hilum (c 

 in A, Fig. 5) is not the geo- 

 metrical centre, but lies 

 considerably nearer to one 

 end. The layers show with 

 varying degrees of sharpness 

 (A} ; between those more 

 strongly can be seen others 

 less strongly denned. To- 

 wards the surface of the 

 grain the layering becomes 

 indistinct. For optical 

 and on account 

 of its lesser density, the 

 organic centre, or hilum, appears somewhat reddish. This is 

 clearest when it is hollow, and it then shows either as a reddish 

 dot, line, cross, or star with dark outlines. The layers imme- 

 diately surrounding the nucleus are developed concentrically; 

 soon, however, the excentricity begins to be produced by the layers 

 thinning off towards one end of the grain, or possibly even in 



FIG. 5. Starch grains from a potato-tuber, A 

 simple grain, B half-compound grain, C and D reasons 

 entirely compound grains, c the nucleus ( x 540). 



