TRANS LOG A TIO V 27 



carried on in this work room. These are assimilation, 

 respiration, and transpiration. By assimilation, the tissues 

 of the plant are built up. The carbon dioxid of the air 

 is taken in through the leaf membranes and combined with 

 water to make starch. This process takes place only in the 

 presence of sunlight and only in the green parts of the plant. 

 The green coloring matter (chlorophyll) is of importance to 

 the growth of the plant, as it absorbs the rays of light; these 

 light rays have the power to split up carbon dioxid 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 shade. The 

 taking up of carbon dioxid 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 dioxid in the air in a crowded city is often double 

 what it is in the country. 



24. Translocation. If the leaves made starch continually 

 during the daylight hours and this starch 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 carrying it to other 

 parts of the plant. This is a part of the process of assimi- 

 lation, 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. 



