KENNETH /'. THIMANN AND CEORGE M. CURRY 661 



a fine, rapidly growing cylindrical cell about 150 fi thick, and extend- 

 ing to 18 cm or more in height, is highly phototropic and has been 

 studied by numerous workers. It diiters Irom the coleoptile in 

 several respects. Firstly, the growing zone, which extends only lor 

 2 to ;") nun below a point a lew tenths of a millimeter from the 

 sporangium, is itself the photoreceptor region, and exposure of other 

 l)arts does not cause curvature. Secondly, there is no evidence for 

 mediation of the growth by a growth hormone, although the spor- 

 angiophores have been shown to contain inciole acetic acid. If drop- 

 lets of lAA solution are applied to one side of the growing zone of a 

 sporangiophore, they do cause curvature away from the drop, but 

 such curvatures, which have been carefully studied in the author's 

 laboratory by Gruen, are no larger than those caused by water alone. 

 Thirdly, the sporangiophore is of course a single cell, very delicate 

 and optically translucent. A suggestive similarity with Avena is that, 

 as Blaauw showed, high light dosages, if given with high intensity, 

 cause a reversal of sign of the phototropic curvature; at about 4000 

 ergs per cm- small negative curvatures result. 



Unlike those of the coleoptile, phototropic curvatures of Phycomyces 

 are not satisfactorily proportional to the light dosage, since brief 

 exposures, even with high intensity, produce only small curvatures. 

 It was found, however, that the phototropism could be quantita- 

 tively studied by exposing the plants to continuous light of known 

 intensity from one side and balancing this with a reference lamp on 

 the opposite side. The plants were thus used as photometers. Using 

 this system and with the same monochromator as used for Avena, 

 the action spectrum for Phycomyces, shown in Fig. 9, has been 

 obtained. 



Fig. 9 shows a main peak at 445 ni/^, a second peak at 470 m/x 

 separated from it by a trough, indications of an inflection at 425 m^u,, 

 and a broad flat peak around 370 m^u,. In all these points, as well 

 as in the steep fall above 480 m^a, it shows a really striking agree- 

 ment with the action spectrum for Avena. With a high degree of 

 probability, the photoreceptors for Avena and Phycomyces in the 

 visible and near ultraviolet are the same. This result is the more 

 notable when one considers the botanical gap that separates Avena 

 and Phycomyces, as well as the structural differences discussed above. 

 It should be added that the sporangiophore, like the Avena coleoptile, 

 contains a carotenoid (Biinning, 1937) and this is limited to the 

 region a few millimeters below the sporangium, as has been shown 

 by direct microspectroscopy (Abbott and Grove, 1959) . 



