PHOTOSYNTHESIS AND RESPIRATION. 81 



117. Does the Leaf gain in Weight by making 

 Starch ? We saw that seedlings grown in darkness lose in 

 dry-weight, while seedlings exposed to light gain in dry-weight. 

 It is easy to prove that the loss and the gain are due chiefly 

 to loss and gain of carbon. How could you prove this by 

 experiment? To what process do you suppose the gain in 

 dry-weight is due when a seedling is grown in the light ? It 

 is very easy to find out. 



(a) Take a plant which has been growing in the light, and set it in 

 darkness for a day or two until a leaf picked off, decolorised, and 

 tested with iodine, shows no starch. Remove a second leaf, kill it by 

 holding it in the steam issuing from a boiling kettle, dry it in an oven, 

 or simply put it in a warm dry place, and weigh it. Now expose the 

 plant to bright light for several hours (from morning until evening) ; 

 remove a third leaf (as nearly as possible similar in size to the second), 

 dry and weigh it, and compare its dry-weight with that of the second 

 leaf. If a Broad Bean seedling or a Tropaeolum plant be used, it will 

 be easy to select leaflets, or leaves, of the same size. 



(6) Repeat the preceding experiment, using leaves severed from the 

 plant and set in water in small bottles or tubes. 



(e) A more accurate result will be obtained if you use pieces of the 

 same leaf for comparison. Use a plant with large leaves, e.g. Sunflower, 

 Primrose, Vegetable Marrow, Rhubarb. If the plant used is growing 

 in a pot or box, keep it in darkness overnight ; if it is growing out-of- 

 doors, you must either get up with the sun to start the experiment or, 

 if practicable, cover the plant in some way the previous evening so 

 that it gets little or no light until you are ready to begin. 



Choose about six symmetrical leaves, divide each in two longitu- 

 dinally, cutting along close to the midrib. Find the area of the 

 removed half-leaves by cutting out a paper model of each half-leaf and 

 weighing the papers against strips of similar paper an inch (or 2 or 3 

 inches) wide, until by balancing you get the total area of the half- 

 leaves. Kill these with steam, dry them, and record the dry weight. 

 Expose the plant to light until evening, remove the remaining half- 

 leaves (cutting along by the midrib), dry, and weigh. 



Experiments of this kind have shown that the leaves of 

 Sunflower and Vegetable Marrow produce in a summer day 

 of fifteen hours about an ounce of starch per square yard of 

 surface. The increase in weight due to starch-formation does 

 not represent the total amount of carbohydrate produced ; in 

 the Sunflower, for example, only one-sixth of the manufac- 

 tured carbohydrate appears as starch, the rest being largely 

 carried from the leaf to other parts of the plant as sugar. 

 s. B. 6 



