LIGHT 



3G1 



illumination of only one edge of the sporangiopliore causes localized 

 extension of that side and consequent curvature. 



This theory, and others of the same general type (32), fail, as Dcl- 

 bruck and Reichardt (86) point out, to account for the extreme local- 

 ization of the receptor which is implied by the unidirectionality of the 

 phototropic curvature; if the entire growing zone were affected, uni- 

 lateral illumination and the known spiral habit of growth would com- 

 bine to generate a helical shape of the sporangiopliore. 



The action spectrum of phototropism in Phy corny ces blakesleeanus 

 (Figure 1) and Pilobolus kleinii has a maximum at about 430-500 m^ 

 (54, 55, 69, 99). This has often been taken as evidence for a carotenoid 

 receptor pigment, carotenoids being present in these fungi and absorb- 

 ing maximally in the same region of the spectrum (297). However, as 

 Galston (107) points out, riboflavin or a derivative of it could with 

 equal probability be the receptor. Carotenogenesis in Phycomyces sp. 

 can be suppressed, at least below detectable levels, without impairment 

 of the phototropic response (241). 



Nor are there any solid data on chemical mediators between the 

 light absorbing system and the growth (or wall extension) response. 

 In higher plants it has been suggested that auxin (indoleacetic acid) 

 is involved, but its role is not yet entirely certain (107). Fungi do 

 produce auxin (Chapter 8), and it is found in fact in the sporangio- 

 phore of Phycomyces nitens (151), but there is at present no reason to 

 accept — and there is considerable evidence against — an auxin theory 

 of phototropism in the fungi (14). 



Phototropism in Pilobus kleinii has been investigated especially by 

 Buller (53), who reviews the earlier literature. In this species a pig- 



Figure 1. The action spec- 

 trum of phototropism in 

 Phycomyces blakesleeanus. 

 Redrawn from Castle (69), hy 

 permission of The Rocke- 

 feller Institute for Medical 

 Research. 



400 500 



Wavelength, m/n 



600 



