REACTIONS OF STARCH. 23 



and the grains show a characteristic more or less symmetrical 

 (according to the symmetry of the grain) dark Maltese cross upon 

 a bright ground, the intersection of the arms of the cross being at 

 the organic centre, the hilum of the grain. If the analyser be 

 rotated the cross will also simultaneously rotate. The grains are 

 therefore doubly refractive, which is taken to indicate a crystalline 

 character, that the grains are of the nature of sphaero-crystals, 

 each lamina being composed of needle-shaped crystals radially 

 arranged. 



General Observations on Starch. In our subsequent plant 

 studies we shall come across starch grains in many forms ; but it 

 will be well to realise that these grains of comparatively large 

 size and more or less distinct lamination are associated with 

 places where starch is accumulated as a reserve store of food. In 

 growing structures generally, in leaves and so forth, starch grains 

 are small, but always to be readily recognised by the iodine 

 reaction, though in some cases it may need to be aided by some 

 process which brings about slight swelling, e.g., very dilute potash, 

 slight warming, or other processes to be learned hereafter. Or 

 if the section in which the starch is be thin, a concentrated 

 solution of iodine in potassium iodide will stain the smallest 

 grains a very deep, practically a black, colour, so that they stand 

 out distinctly amidst the brown protein substances. 



In the processes of germination starch undergoes changes, 

 being gradually converted into glucose or maltose, and then 

 dissolved. In the living plant this probably takes place from 

 without inwards, the solution either being regular, or local 

 corrosion goes on so that surface hollows, or pits, or even fine 

 canal-like openings extending even to the centre of the grain, are 

 formed. Such corroded grains may be found occasionally in 

 germinating seeds or potatoes, or may be produced by the action 

 upon starch of a solution containing a special body, called 

 " diastase". This solution may be prepared by an aqueous solu- 

 tion of commercial diastase with about -05 per cent, of citric acid 

 added, or by dissolving a little malt extract in water ; for diastase 

 is contained in, and can be extracted in large quantity from, 

 malt or germinating barley. Starch grains which are thus being 

 changed in the processes of germination lose in part or wholly 

 the power of taking a blue coloration with iodine ; they may 

 show a partial or more complete red coloration, or even a nearly 

 pure yellow colour. In this latter case we have what is called 



