246 TEXTBOOK OF BOTANY 



chlorophyl-containing cells are found chiefly in the leaves ; 

 the leaves are, therefore, among other things, the car- 

 bohydrate-making organs of the plant. They obtain carbon 

 dioxid directly from the air, or from water if they live in 

 water ; the water that is needed is brought to them from the 

 soil by means of -the root hairs and the conducting tissues of 

 the root and stem. The first form which the newly-made 

 carbohydrates take is probably that of some of the simpler 

 sugars such as glucose. It is in the making of sugars out of 

 carbon dioxid and water that the work of the chlorophyl is 

 done. Much of the sugar, as we shall see, is changed to 

 starch, but with this change the chlorophyl has nothing to do. 



The sugar, once formed, may be handled by the plant in 

 any one of several ways. It may be used at once, in com- 

 bination with some of the substances that the plant receives 

 from the soil, in building up more complex materials, such 

 as proteins and finally living matter. It may be broken 

 down in the process of respiration, either in the cell in which 

 it was formed or in another cell to which it has been passed 

 on, so giving up its store of energy for use in doing some of the 

 work of the plant. Or if, as is likely to be the case on a bright 

 day, sugar is made more rapidly than it can be used, it may 

 be stored away for later use. The carbohydrate food is 

 usually not stored in the form of sugar, at least in the cell in 

 which it was manufactured. Sugar, being soluble in the cell 

 sap, would make the sap more and more dense and sirupy as 

 its manufacture goes on. This would interfere with the 

 movement of materials within the cell and possibly with 

 other forms of activity of the living matter. For these and 

 perhaps for other reasons, sugar is not usually a convenient 

 form for the storage of large amounts of food. 



As a rule, when the solution of sugar in the cell sap has 

 reached a certain strength, some of it is changed into starch 

 and stored in the form of small grains inside the chloroplasts. 

 Starch is a compact form of food, and being insoluble it does 



