TRANSLOCATION 27 



the growth of the plant, asit absorbs the rays of Hght. These 

 light rays have the power to split up carbon dioxide into its 

 parts, carbon and oxygen, so that the plant can utilize the 

 carbon and set free the oxygen. This process wholly ceases 

 in darkness, and proceeds much more slowly on dark days or 

 in shady locations than in full sunlight. This explains why 

 most plants grow better in the open than in the shade. The 

 taking up of carbon dioxide and giving off of oxygen also 

 accounts in a measure for the purer air of country districts 

 where trees and growing things abound. The percentage of 

 carbon dioxide in the air in a crowded city is often double 

 that in the countiy. 



24. Translocation. If the leaves made starch contin- 

 ually during the daylight hours and it remained where it 

 was manufactured, they would soon become thick and bulky 

 and their stems would be unable to support them. As in 

 most good factories, however, a means is provided of taking 

 the manufactured product and carr^dng it to other parts 

 of the plant. This is a part of the process of assimilation, 

 and is called translocation. It takes place during the hours 

 of darkness. Starch itself is not directly soluble in water, 

 but the leaf cells contain substances called enzymes, which 

 change the starch to sugar, and, as every one knows, sugar 

 is readily soluble. The sugar is then taken up in the sap 

 and carried to the stem or seeds or roots, where it is stored. 



Many plants which live from year to year store large 

 quantities of food in their roots over winter and are thus 

 able to start into growth very early in the following 

 spring. If no leaves are produced, no starch can be made 

 and hence none can be stored in the roots. This fact sup- 

 plies us with an excellent method of fighting weeds like quack 

 grass and Canada thistle, which are serious weed pests largely 

 on account of the food they store and the resulting vigor 

 of their growth. If these plants are prevented from reaching 

 the light by continuous cultivation, they will be unable 



