STARCHES. 69 



equal on all sides, but afterwards became much greater on one 

 side than on the other. This view is borne out by the fact that 

 in young grains the hilum is central. Some of the lines or stria- 

 tions are more strongly developed than others, and some of the 

 grains show scarcely any lines. Nearly all the grains are 

 single, but some double grains may be found containing two 

 nuclei, each with concentric and eccentric markings about it 

 and a distinct dividing line. Some grains are not double, but 

 contain two nuclei, i. e., are bi-nucleated. 



The nucleus is usually a circular spot in the potato grain, 

 but sometimes it is fissured. The small end of the grain, where 

 the nucleus is found, is thicker than the broad end, the grain 

 being shaped somewhat like a clam shell. To observe the exact 

 shape of the grains they must be seen from all sides. This is 

 readily done by pressing gently on the cover glass with a needle, 

 when the grains will roll over and may be seen in various posi- 

 tions. Another way to cause the grains to roll is to place a 

 drop of alcohol at the edge of the cover glass, which, by mixing 

 with the water, causes currents. When thus observed, the 

 grains are seen to be somewhat flattened (Fig. 43, II). 



Let a drop of 5 per cent, caustic potash solution run under 

 the cover glass and watch the starch grains as the alkali comes 

 in contact with them. They swell, and at first the layers be- 

 come more distinct, but after a while they grow less distinct 

 and finally disappear. The more watery layers at first absorb 

 water under the action of the potash more rapidly, and thus 

 stand out more distinctly. Finally the whole grain dissolves 

 and disappears. Other reagents which cause starch to swell 

 and dissolve are concentrated solutions of chloral hydrate, zinc 

 chloride, calcium chloride, strong hydrochloric and sulphuric 

 acids, oil of cloves. 



Mount some potato starch in water and apply a gentle heat 

 near the edge of the cover glass until the grains there become 

 translucent, then quickly remove the flame and cool the slide. 

 Compare the grains that have been affected by heat with those 

 that have just begun to change and those that are still intact. 

 Heat causes the grains to swell and ultimately to assume a 

 gelatinous form. Intermediate granular and translucent 

 stages occur in those grains that have not been heated too 

 strongly. The temperature of gelatinization varies appreciably 

 with the kind of starch. 



Examine potato starch mounted in glycerin, also in oil of 

 cloves. The grains appear brighter and the lines are almost 

 invisible. If an air globule be imprisoned in the hilum it will 

 appear as a dark spot. Glycerin and oil of cloves and any 

 similar liquids are unsuited as a medium for mounting starch 

 because of their high refractive power, which causes details of 

 structure to be almost invisible. 



