III.] 



THE CARBOHYDRATES. 



17 



1. I. Starch {C^Jl^fi■^),^. — The n in this case is not less than 4, 

 and may be 10 or 20; indeed, Brown and Heron suggest the for- 

 mula ioo(Cj2H2oOjq). Starch is one of the most widely distri- 

 buted substances in plants, and it may occur in all the organs of 

 plants, either (a.) as a direct or indirect product of the assimila- 

 tion of CO2 in the leaves of the plant, or (6.) as reserve material 

 in the roots, seeds, or shoots for the later periods of generation 

 or vegetation. 



Preparation. — Wash a potato tlioroughly, and grate it on a grater into 

 water in a tall cylindrical glass. Allow the suspended particles to subside, 

 and after a time rote the deposit ; the lowest stratum consists of a white 

 powder or starch, and above it lie coarser fragments of cellulose and other 

 matters. Decant off the supernatant fluid which becomes brown on 

 standing. 



(a.) Microscopical Examinatioa — Examine the white deposit 

 of starch, noting that each starch-granule shows an eccentric hilum 



Fig. 3. — Potato Starch. 



with concentric markings (fig. 3). Add a very dilute solution of 

 iodine. Each granule becomes blue, while the concentric markings 

 become more distinct. 



(h.) Compare the microscopical characters of other varieties of starch — e.g., 

 rice, arrowroot, &c. Each granule consists of an outer layer of cellulose en- 

 closing alternating layers of granulose and cellulose, so that they ])resent a 

 laminated ajjpearance. There are very great varieties in the shape and size 

 of starch grains. 



(c.) Squeeze some dry starch i)owder between the thumb and forefinger, and 

 note the peculiar crepitation sound and feeling. 



{d.) Polariscope. —Examine starch granules with a polarisation microsco})e. 

 With crossed Nicol's, when the field is dark, each granule shows a dark cross 

 on a white refractive ground. They are doubly refractive. If a plate of mica 

 be placed on the stage of the microscope under the starch grains, the latter, 

 with polarised light, exhibit interference colours (fig. 4). 



2. Prepare a Solution.— Place i gram of starch in a mortar, 

 rub it up with a little cold water, and then add 50 cc. of boiling 

 water. Boil until an opalescent imperfect solution is obtained. 



