GROWTH SUBSTANCES FOR GEOTROPISM 173 



of geotropic sensitivity in the coleoptile of Avena sativa, Hordeum 

 vulgare, and Phalaris canariensis were carried out by von Gntten- 

 berg (1912). It was found that a short tip region in these plants 

 (3 mm. in Avena and 4 mm. in Hordeum and Phalaris) is far 

 more sensitive than the lower regions of the coleoptile, though the 

 basal region is not entirely insensitive. Dolk (19296) came to a 

 similar conclusion by another method and found that the length 

 of the sensitive zone in the Avena coleoptile is about 5 mm. 



From what has been said, it is clear that geotropic as well as 

 phototropic sensitivity is localized in the tip of the Avena 

 coleoptile. However, there seems to be a difference between the 

 two loci of stimulation, since phototropic sensitivity is mainly 

 confined to the upper 0.5 mm. and geotropic sensitivity to the 

 upper 3 to 5 mm. 



Transmission of the Stimulus. — The geotropic stimulus is 

 transmitted from the tip into the basal region just as is the photo- 

 tropic stimulus. Investigations on the pathway of stimulus 

 conduction were carried out by Boysen Jensen using methods 

 described in the section on transmission of the phototropic 

 stimulus. When a mica plate is inserted into a transverse inci- 

 sion made upon the upper side of a horizontal Avena coleoptile, a 

 strong geotropic curvature results; but when the incision faces 

 downward, the curvature is very weak. From this kind of experi- 

 ment it can be concluded that the stimulus is transmitted upon 

 the underside of the coleoptile. The problem was investigated 

 also by Purdy (1921) ; she found that after the stimulation due to 

 woimding had subsided, the size of the negatively geotropic 

 curvature yielded a d value of 1.73 mm. when the incision faced 

 upward and a d ^•alue of 0.74 mm. when the incision faced down- 

 ward. It is obvious that the stimulus transmission is far stronger 

 on the lower side, though a significant conduction of stimulus can 

 be demonstrated upon the upper side (Fig. 51). 



Since the basal region of the grass coleoptile, as well as the tip 

 zone, is affected by gravity, any explanation of curvature that 

 comes about in response to geostimulation of the tip must take 

 into consideration also the direct effects of gra\aty upon the basal 

 region. Geotropic experiments are probably not so satisfactory 

 as those dealing with phototropism. Zollikofer (1926) showed 

 that transport of growth substance from the coleoptile to the first 

 internode in grass seedlings can be retarded by burning the first 



