THE MOLECULAR FORCES IN PLANTS. 115 



slide. We now place at the edge of the cover-glass a drop of some 

 Iodine reagent (Iodine water, iodised Potassium iodide solution, 

 or dilute iodised alcohol). To prepare Iodine water we treat 

 Iodine with distilled water, and put aside for a few days. To 

 prepare the iodised Potassium iodide solution we add 60 parts 

 of water to 3 parts of Potassium iodide, and then add 1 part 

 of Iodine. The solution may be diluted with water. The Iodine 

 reagent, being placed at the edge of the cover-glass, gradually 

 advances towards the starch grains, arid it may easily be observed 

 under the microscope that these at first stain faintly bluish, but 

 little by little become more intensely blue as they take up more 

 and more Iodine. 



Dry potato starch is placed on a slide in a drop of Iodine tinc- 

 ture, freshly prepared by dissolving Iodine in absolute alcohol. 

 On microscopic examination it is seen that the starch grains do 

 not become blue, but brownish in colour. If we allow access of 

 water, the characteristic blue tint appears. The starch grains, 

 therefore, are only able to stain blue with Iodine when they have 

 imbibed a fair quantity of water. 



45. Behaviour of Starch Grains in Polarised Light. 



It is very instructive to investigate the phenomena studied by 

 von Mohl 1 and Nageli, 2 which are exhibited by starch grains 

 when examined in polarised light. We require for this purpose 

 a polarising apparatus and a microscope stand with a sufficiently 

 high stage. The polariser is accommodated below the stage. For 

 the analyser it is best to use the ocular analyser of Abbe, which, 

 as also the polariser, may be obtained of Zeiss, Jena. 



The starch grains are mounted on the slide in the usual 

 manner, in a drop of water, and covered with a cover-glass. 

 When the planes of polarisation of the polariser and analyser are 

 parallel to one another, the field of view is bright, and they must 

 be so arranged for focussing the preparation. When the Nicols 

 are crossed (by rotating the ocular analyser), the field of view 

 becomes dark. The amylum grains stand out very brightly from 

 the dark background, and bear a black cross. Very beautiful 

 colour effects are to be observed, if we examine starch grains 

 through a polarising microscope after plates of gypsum of a par- 

 ticular description have been interposed between the object and 

 the polariser. 3 



