58 STRUCTURE AND PHYSIOLOGY OF SEED PLANTS 



might obscure the starch test. Heat the alcohol in a water bath 

 away from any flame. Place the leaves for ten or fifteen minutes 

 in a solution of iodine, rinse off with water, put in a white plate 

 or saucer, and note what portions of the leaf, if any, show the 

 presence of starch. 



Ebfeeences. Detmer-Moor, 9 ; Ganong, 10 ; Darwin and 

 Acton, 11 ; Pfeffer-Ewart, 31, I. 



EXPERIMENT XXXIII 



Consumption of starch in nasturtium (Tropaeolum) leaves.* * 

 Select some healthy leaves of TropoBolum on a plant growing 

 vigorously indoors, or, still better, in the open air. Shut off the 



sunlight from parts of the selected 

 leaves (which are to be left on the 

 plant and as little injured as possible) 

 by pinning circular disks of cork 

 loosely on opposite sides of the leaf, 

 as shown in Fig. 3. On the afternoon 

 of the next day remove from the plant 

 these leaves and (for control purposes) 

 some others to which no cork disks 

 were attached. Treat all as described 

 in the preceding experiment, taking 

 especial pains to get rid of the chlorophyll by changing the 

 alcohol as many times as may be necessary. What does this 

 experiment show in regard to the consumption of starch in the 

 leaf ? What has caused its disappearance ? * 



It may be fairly taken for granted that if the leaf contained any 

 starch when the corks were pinned onto it, all parts of it were 

 somewhat equally full of starch. If the experiment results in 

 showing absence of starch in the part deprived of light by the 

 cork, it may be thought that starch manufacture was stopped 



1 Or put a plant with starch in the leaves in a moist chamber without light 

 for a day or two, with cut-ofP leaves beside it. Then test the attached leaves and 

 the cut-off ones for starch. Explain results. 



Fig. 3. Leaf of Tropmolum 

 partly covered with disks of 

 cork and exposed to sunlight 



