362 THE MICROSCOPE AND ITS REVELATIONS. 



crease in size, the chlorophyll thins itself out as a mere covering film, 

 and at last disappears altogether. So long as the starch-grains remain 

 imbedded in the protoplasm-layer, they continue to grow; but when they 

 accumulate so as to occupy the cell-cavity, their growth stops. They 

 are sometimes minute and very numerous, and so closely packed as to 

 fill the cell-cavity (Fig. 246); in other instances they are of much larger 

 dimensions, so that only a small number of them can be included in 

 any one cell; while in other cases, again, they are both few and minute, so 

 that they form but a small proportion of the cell- con tents. Their nature 

 is at once detected by the addition of a solution of Iodine, which gives 

 them a beautiful blue color. Each granule, when highly magnified, ex- 

 hibits a peculiar spot, termed the hilum; round which are seen a set of 

 circular lines, that are for the most part concentric (or nearly so) with it. 

 When viewed by Polarized light, each grain exhibits a dark cross, the point 

 of intersection being at the hilum (Fig. 247); and when a Selenite-plate 

 is interposed, the cross becomes beautifully colored. Opinions have been 

 very much divided regarding the internal structure of the starch-grain; 

 but the doctrine of Nageli, 1 that it is composed of successive layers which 



- 247 



Cells of Pceony, filled with starch. Granules of Starch, as seen under 



Polarized light. 



increase by ' intussusception,' is the one now generally adopted. These 

 layers differ in their proportion of water; the outermost layer, which is 

 the most solid, having within it a watery layer; this, again, being suc- 

 ceeded by a firm layer, which is followed by a watery layer; and so on, 

 the proportion of water increasing towards the centre in both kinds of 

 layers, and attaining its maximum in the innermost part of the grain 

 where the formation of new layers takes place, causing the distention of 

 the older ones. Although the dimensions of the Starch-grains produced 

 by any one species of plant are by no means constant, yet there is a cer- 

 tain average for each, from which none of them depart very widely; and 

 by reference to this average, the starch-grains of different plants that 

 yield this product in abundance may be microscopically distinguished 

 from one another, a circumstance of considerable importance in com- 

 merce. The largest starch-grains in common use are those of the plant 

 (a species of Canna) known as Tons Us mois; the average diameter of 

 those of the Potato is about the same as the diameter of the smallest of 

 the Tous les mois; and the size of the ordinary starch-grains of Wheat 

 and of Sago is about the same as that of the smallest grains of Potato- 

 starch; while the granules of Rice-starch are so very minute as to be at 

 once distinguishable from any of the preceding. 



1 See his papers in " Sitzungsberichte der Kon. Bayer. Akad. der Wissen- 

 schaften," 1862 and 1863; and Sachs' "Handbook of Botany" (Bennett's Transla- 

 tion), pp. 56-62. 



