STARCHES. 67 



CHAPTER XV. 



STARCHES. 



The green color of the leaves of plants is due to a colored sub- 

 stance called chlorophyll, which is diffused through certain 

 proteid granules in the cells. The function of this substance 

 is to utilize the energy of the sun's rays in converting carbon 

 dioxide, the main food material of plants, into some form of 

 carbohydrate. Starch is a carbohydrate, but that formed by 

 the green coloring-matter from carbon dioxide is not starch. 

 The carbohydrate in question is combined with nitrogen and 

 sulphur (taken up by the plant in the form of salts) into a pro- 

 teid by the vital processes of the cells. It is not known what 

 the composition of the carbohydrate is nor the processes in- 

 volved in building up the proteid substance. 



The starch granules found in chlorophyll bodies were at one 

 time supposed to be formed directly from carbon dioxide, but 

 Strasburger has clearly shown that this is not true. They 

 result from a breaking down, by the chlorophyll bodies, of proto- 

 plasm previously formed by those bodies. Starch, hence, is a 

 result of a destructive process. It is probable also that the 

 starch formed by amyloplasts in cells devoid of chlorophyll is 

 also formed from proteids. 



There are various reserve food materials found in the plant, 

 and starch is one of the most important. It is found in various 

 parts of the plant, for example, the stems of certain palms, 

 which are gorged with it ; it is the principal substance in tap- 

 roots, root-stocks, corms, bulbs, tubers; many fruits and seeds, 

 as grains, pulses, bananas. 



The power of building up protoplasm from starch is possessed 

 by all living cells, whether possessing chlorophyll or not, and 

 independent of sunlight, but no new carbohydrate is ever 

 formed without light. A tuber will sprout and grow in the 

 dark until all the starch is used up, when growth ceases, and to 

 renew growth it must be brought into light. 



DESCRIPTION OF STARCH GRAINS. 



They are hard and of various sizes and often possess shapes 

 and markings sufficiently characteristic to identify the plant 

 from which they come. They vary from 1 to 100 or even 200 

 microns. A micron or micromillimeter is one-thousandth of a 

 millimeter and is represented by the sign //, the Greek letter m. 

 Starch gains are simple or compound. Compound grains con- 

 sist of two or more simple grains united to form larger grains. 

 Potato, wheat, arrow-root, corn and ginger starch are examples 



