GROW TH OF PLANTS 



153 



in proportion to tlie intensity of light; therefore, plants in open 

 spaces are less sensitive to light than those growing in the shade, 

 while the most sensitive are those that have grown in total 

 darkness. The latter are, as a rule, used for precise phototropic 

 experiments. These experiments are carried out in the photo- 



FiG. 42. — The leaf mosaic of an ivy (after Lubimenko). 



tropic chamber, a tight box having the interior painted black 

 and for experimental purposes arranged to admit light through 

 a small opening in one of the walls (Fig. 43). 



The seedlings of some cereals, particularly those of sorghum 

 and millet (Setaria), are most convenient objects for such 

 experiments. These seedlings 

 consist of a rather long hypocotyl 

 bearing a comparatively small 

 coleoptile, which encloses the first 

 true leaf. If the shoot is placed 

 in a phototropic chamber, the 

 hypocotyl will form a distinct 

 curve, and the tip of the coleoptile 

 will be directed toward the light . .„.,,,, 



,^. ,,. * . /^ ^ xi J- Fig. 43. — Sporangia of Piloholus 



(Fig. 44). At first the curvature ^^j.ned toward the openings of a 



is formed directly beneath the tip phototropic camera {after Smith 

 of the coleoptile, but later, as 



further growth of the hypocotyl takes place, the curvature is 

 in a somewhat lower region. By means of simple experiments, 

 it may be easily shown that the coleoptile is the organ that 

 perceives light, while the hypocotyl merely responds to the 

 stimulus transmitted from the coleoptile. If an opaque hood 



