164 GROWTH HORMONES IN PLANTS 



phototropic stimulus and response apply equally well to photo- 

 tropism in other plants. Many valuable contributions to our 

 knowledge of phototropic curvature, particularly in recent years, 

 have developed out of studies on the response of dicotyledonous 

 shoots to light. 



Distribution of Phototropic Sensitivity. — Darwin found long 

 ago that only the tip of the grass coleoptile was sensitive to light 

 and that this apical region determined the phototropic curvature 

 of the lower portion. Similar results were obtained also with 

 the stems of Brassica oleracea. However, Darwin's conclusions 

 were not entirely convincing because of the considerable variabil- 

 ity in his experiments. Later on, Rothert (1894) came to differ- 

 ent conclusions regarding the phototropism of stems of Brassica 

 napus, Agrostemma, Vicia, and other dicotyledonous plants. 

 The sensitivity to light was found to be irregularly distributed 

 in the stem, being particularly strong in a relatively short region 

 near the tip and weaker in the basal region. This differential 

 distribution of light sensitivity was found to decrease with age in 

 Vicia. In other seedling stems, such as those of Tropaeolum, 

 Solanum, and Coriandru.m, the sensitivity appeared to be 

 regularly distributed over the stem, while the condition in 

 Daucus and Linum formed a transition between these two groups. 



Transmission of Phototropic Stimulus. — Rothert demonstrated 

 the transmission of a phototropic stimulus in various seedling 

 stems of dicotyledonous plants. When the upper region of the 

 stem of Brassica napus was unilaterally lighted, and the lower 

 region was darkened by dry earth, paper aprons, or paper tubes, 

 phototropic curvatures appeared in the darkened region of the 

 stem. Similar results were obtained with Agrostemma, Tropae- 

 olum minus, and some other species. It was relatively difficult 

 to demonstrate any transmission of stimulus in Vicia. A study 

 of Rothert's photographs for the conduction of the phototropic 

 stimulas shows that the curvatures which are obtained in the 

 darkened basal region in dicotyledonous seedlings are smaller 

 than those obtained in Avena similarly treated. It appears, 

 therefore, that the region over which the stimulus is transmitted is 

 comparatively small in these plants. A decided phototropic 

 stimulus transmission, extending over several centimeters and at 

 times of marked strength, has been demonstrated in the stems of 

 Linum usitatissimum, Brodiaea congesta, and Galium purpureum. 



