20 I. THE COMPOUND MICROSCOPE STRUCTURE OF STARCH. 



the lamination to be recognised. Very frequently we see a 

 colourless vesicle adhering to the side of the grain (A), the wall of 

 which is referable, however, not to the sub- 

 stance of the starch grain, but to the plasma 

 mass adhering to it. 



It must strike the observer that the small 

 latex spherules distributed in the water are in 

 tremulous motion. This is the so-called Brown's 

 molecular movement, or " Brownian move- 

 in ooh ment," which we can therefore take this oppor- 



r IG. ll>. oiaiLii j . 



grains from the latex tunity of learning to recognise, and which is 

 not a phenomenon due to life, but is referable 

 P erha P s to fine streams in the fluid carrying 

 with them the minute globules. 

 Iodine Reactions of Starch. Having learned this much about 

 the form and structure of starch grains, we will now produce 

 some reactions upon them, and study the results under the micro- 

 scope direct. First take a preparation of potato starch again out 

 of the moist chamber. After we have focussed the microscope 

 upon the grains, place a drop of a solution of iodine (in water, 

 alcohol, or in watery iodide of potassium) at the edge of the 

 cover-glass. For this purpose is specially recommended the use 

 of a potassium -iodide iodine solution which is prepared by treat- 

 ing 0-5 gram potassium iodide and 1 gram iodine with a little 

 water, then diluting with water to 100 c.cm., and leave it to 

 stand on the iodine which separates out. 1 In using the reagent 

 we must take special care that the drop does not run over the 

 cover-glass, and thence, perhaps, upon the objective. If a drop 

 runs upon the cover-glass, let it be immediately sucked off with 

 blotting-paper. If the reagent reaches the objective, plunge the 

 lower lens of this latter into pure water, or spray it with water 

 from the wash bottle, and clean it afterwards with the piece 

 of linen rag, or Japanese rice-paper, already recommended for 

 cleaning purposes. 



1 I have prepared this form (Arthur Meyer's) of the well-known reagent 

 by grinding 0'5 gram potassium iodide and 1 gram iodine together in a 

 glass mortar, and dissolving in 1 c.c. water. If the potassium iodide solution 

 is made weaker, as, e.g., with 3 c.c. water, the iodine is not completely dis- 

 solved. The result, after addition of the 99 c.c. water, is to produce a weak 

 potassium iodide solution which is quite saturated, possibly even super- 

 saturated, with iodine, and in which metallic iodine is separated out in 

 exceedingly fine particles. [Eo.] 



