The Food Making of Plants 95 



Besides glucose, fructose, also consisting of ^^\<fy^ but 

 having a different arrangement of atoms in the molecule, and 

 cane sugar ^^^^^w are of frequent occurrence in plants. 



After the action of light has brought about the formation 

 of sugar (or if a sugar solution is supplied to a plant) the 

 further steps in assimilation by which nitrogen, sulphur, and 

 phosphorus are combined with carbon, hydrogen, and oxygen 

 to form protoplasm, can take place in darkness. The process 

 is a complex one and by no means fully understood. 



The accumulation of starch is a sign that carbon assimila- 

 tion is going on, although in many monocotyledons the food is 

 carried away so rapidly that no starch is accumulated in the 

 leaves. 



Much of the food stored in seeds is starch. Remove a 

 mealie seedling from the soil. Cut the seed in two. Cut off 

 a portion of the stem about half an inch long just above the 

 seed. Cut this piece in two lengthwise. Place in a test-tube 

 containing an inch or two of water. Gradually add iodine. 

 Portions of the seed will show blue where some starch is still 

 left. The stem does not stain blue. The food cannot pass 

 to the growing parts as starch ; it has been changed to sugar. 

 The bundles through which the sap is passing up into the 

 leaves are stained a yellowish brown. In the North American 

 Maple, the sap is so filled with sugar when it is passing up into 

 the buds in spring, that it is drawn away through little troughs 

 placed in holes bored into the trunk as far as the new wood. 

 On a bright day a drop falls about once a second. Drop by 

 drop about twenty-five gallons of sap may flow from one tree 

 in a season, until the buds begin to unfold, and this will boil 

 down to about five pounds of sugar. 



Ex. 40. How can we tell when a plant is making starch ? Place 

 in ajar of water a plant which grows in water. The green silky thread- 

 like plants in ponds are suitable. In spring, leaf-bearing plants may be 

 obtained in some ponds which should be used when possible. Place the 

 jar in the sun. In a few minutes bubbles will rise from the plant. Place 

 the plants in water which has been boiled. No bubbles are given off. 

 Boiling the water has driven off a gas which the plants need in making 

 starch. Breathe through a glass tube into some boiled water, and place 

 the plants in this water. Bubbles will soon begin to come off. Carbon 



