GROWTH SUBSTANCES FOR PHOTOTROPISM 137 



ter. In these experiments, transverse incisions were made upon 

 the front (lighted) or back (shaded) side of the coleoptile a few 

 milHmeters below the tip. In some cases, mica plates were 

 inserted in the cuts. Then the tip was unilaterally illuminated 

 above the incision, and it was determined whether the cut had 

 interfered with the conduction of the stimulus to the growing 

 portions below. Boysen Jensen's experiments showed that the 

 stimulus is conducted on the back (shaded) side of the Avena 

 coleoptile. Later, Purdy (1921) carried out quantitative studies 

 deahng with the localization of the stimulus conduction. In 

 order to eliminate the stimulus resulting from making the incision, 

 the experimental plants were used 24 hours after the operation, 

 when they were again entirely straight. Pieces of mica were 

 then inserted into the incisions, and the tips of the plants were 

 unilaterally illuminated, with the pieces of mica facing either 

 toward the front or the back. The inconsequential curvature, 

 which was caused in some oases by the insertion of the mica 

 pieces, was measured and taken into account. When the incision 

 was on the front side, a phototropic curvature resulted with a 

 d value (see Chap. II) of 0.61 mm.; in other cases, of 0.10 mm. 

 (Fig. 45). Purdy concluded from her experiments "that the 

 strongest tendency is for transmission of the stimulus to take 

 place in a longitudinal direction, mainly locaHzed in the side of 

 the coleoptile farthest from light." 



The importance of the vascular bundles in conduction of the 

 phototropic stimulus from the tip downward has been indicated 

 in the experiments of Baffet et al. (1933) with glass plates inserted 

 transversely in the Avena coleoptile. 



Beyer (1928a) tried to show that conduction of the phototropic 

 stimulus can take place upon the front as well as upon the back 

 side. In his experiments, the basal portion of the plants were 

 darkened with sand. According to Cholodny (1929a), sand is 

 translucent and is not sufficient to protect the basal portion of the 

 coleoptile entirely from light (see also Reinhard and Bro, 1933). 



Conduction of a stimulus upon the front side of the coleoptile, 

 although a weak one, has been repeatedly observed, first by Purdy 

 and later by others. From the standpoint of the growth-sub- 

 stance explanation, such a phenomenon might be expected, but 

 the stimulus transmission that takes place upon the front side is of 

 an entirely different nature from that occurring on the back 



