AN INTRODUCTION TO PLANT BIOLOGY 



103 



Another way of showing the effect of light on starch-formation 

 is as follows : Take a piece of sheet cork, or thick pasteboard, or 

 tin-foil, about two inches square, and cut out a triangle, star, or any 

 figure preferred, from the center. By means of pins or paper clamps 

 fasten this on the upper side of a leaf of a potted plant, and a piece 

 of black cloth or sheet of cork on the lower side ; or cut the opening 

 desired in a strip of tin-foil and then fold this 

 around the middle of the leaf. Set the plant in 

 bright sunlight with this leaf supported so that the 

 upper side will get full illumination. Near the 

 close of the day, cut off the leaf, dip into boiling 

 water, then place in alcohol for a day or two. 

 Finally, test for starch with the iodine (or chloral 

 hydrate and iodine) as in above experiment. 

 Make a sketch of the leaf, and by colored pencil 

 or shading indicate where starch was formed. 

 Label the portions of the leaf '* exposed to light " 

 and "in dark.'! 



FIG. 38. The darker 

 ends of this leaf 

 were colored by 

 iodine, indicating 

 starch. The 

 center was pro- 

 tected from sun- 

 light. 



Variegated Leaves. Many varieties of 

 plants grown in greenhouses have " varie- 

 gated " leaves with large white areas due to 

 the absence of chlorophyll. That starch is 

 not formed in these areas can be shown by 

 taking a leaf which has been in sunlight all 

 day and testing for starch as in the above 

 experiments. Plants grown for a long time 

 in a dark room lose their chlorophyll (e.g., sprouting potatoes 

 in a cellar), and after a day in sunlight their leaves show 

 no starch. 



Intensity of Light Required. It must not be inferred from 

 the preceding experiments that light must necessarily be 

 in the form of bright and direct sunlight. Starch-formation 

 is most rapid in direct sunlight, but probably goes on in 

 light of all intensities, even in moonlight. This explains 

 why some species of plants can live and grow slowly in 

 shaded spots in the woods; they apparently make use of 

 the diffuse and weak light which reaches them. Moreover, 



