AT OUR HOMES 231 



beans, and the gluten of grains, there is formed the complex 

 compound known as protein. 



SUMMARY 



Starch arid cellulose are made up of the elements carbon, hydrogen* 

 and oxygen, and these are in'the same proportions by weight in both 

 substances. Ordinarily this would mean that any two bodies of which 

 this is true are one and the same substance. But starch and cellulose 

 have entirely different properties and are recognized as entirely sepa- 

 rate substances. While starch is one of the chief foods of mankind, 

 cellulose is indigestible and is familiar to us in such forms as cotton and 

 linen (flax) fibre, wood, paper, etc. 



The most reasonable explanation of the production of different 

 substances from the same proportions by weight of the same elements 

 is that the ways in which the atoms have been put together must be 

 unlike. In the studies of chemical compounds of plant or animal 

 origin cases of this kind are not uncommon. 



Plants under the influence of sunlight can manufacture their own 

 food as starch and sugar out of the raw materials of water and carbon 

 dioxide gas. They may store any excess of it in root, stem, leaf, seed, 

 or fruit. Probably it is in the form of sugar only that it is conveyed 

 in the sap of the plant from where it is made to where it is stored, and 

 then again to where it is needed later for the growth of the plant and the 

 maturing of its seeds. 



The accomplishment of this change back and forth of starch and 

 sugar, and their change into cellulose in the woody part of the plant 

 structure, is not well understood by either botanists or chemists. The 

 substance made by the plant known as diastase is probably concerned 

 in these chemical changes in the life processes of the plant. 



In some plants along with the starch stored as food for the plant is 

 more or less of vegetable oils. Olive, cocoanut, cottonseed, and lin- 

 seed (flaxseed) oils are good illustrations. Many different kinds of 

 nuts contain oil enough to be very noticeable when the pulp is crushed. 



The presence of starch, especially after having been boiled in a little 

 water, is made known by a characteristic blue coloration when treated 

 with a little weak iodine solution. (See page 301 for test for proteins.) 



When a few drops of Fehling's solution, whose blue color is due to 

 the presence in it of a compound of copper, is added to a solution con- 

 taining any grape sugar or glucose and heated, the blue color disappears 



