PHOTOTROPISM 475 



papers on light responses in fungi will be mentioned, because they have 

 a direct bearing on the theory of phototropism. 



Castle (1933) investigated the relation between light refraction and 

 absorption and the direction of phototropic curvatures in great detail. 

 Blaauw (1914) found that the growth of Phycomyces sporangiophores is 

 accelerated by light. This seemed to contradict the explanation of the 

 positive curvature produced by light; it means that the back is growing 

 faster than the front. However, Blaauw had already pointed out that this 

 could be explained by the lens action of the completely transparent 

 sporangiophore, which focuses the light on the rear, thereby causing an 

 acceleration of growth. Buder (1920) then tried immersing Phycomyces 

 sporangiophores in paraffin oil and obtained negative curvatures. This, 

 then, is a perfect proof that Blaauw's mechanism holds for phototropic 

 curvatures in Phycojnyces. The refinement in calculation made by 

 Castle also confirmed Blaauw's views. 



Van der Wey (1929) discovered a rather exceptional case of phototropic 

 behavior in Pilobolus. The sporangiophores of this phycomycete shoot 

 off their sporangia in the direction of light. The accuracy of this photo- 

 tropic aiming can be measured by observing the spread of the sporangia 

 that are shot against a glass plate fixed in front of a light source. When 

 light hits the sporangiophores from two different directions, they aim at 

 the arithmetical center between the light sources. When there is a 4° 

 divergence between the light sources, half the sporangiophores aim at one, 

 half at the other. From about 8° on, there is a complete separation in 

 direction of curvature. Van der Wey tries to explain this by the assump- 

 tion that the black sporangia intercept the light rays in such a fashion 

 that no light reaches the ring of yellow pigment at the base of the spo- 

 rangiophore swelling. Biinning (1937) has shown that light absorbed by 

 this orange pigment is responsible for the phototropic response. There- 

 fore any deviation of the sporangiophore from the direction of the inci- 

 dent light will cause the light to be absorbed and produce a growth 

 response. 



REFERENCES 



Arisz, W. H. (1915) Untersuchungen iiber den Phototropismus. Rec. trav. botan. 

 neerl., 12: 44-216. 



Asana, R. D. (1938) On the relation between the distribution of auxin in the tip 

 of the Avena coleoptile and the first negative phototropic curvature. Ann. 

 Botany London, (N.S.)2: 955-957. 



Bandurski, R. S., A. W. Galston, R. S. Baker, and D. S. Robertson (1950) Photo- 

 tropic sensitivity of coleoptiles of albino corn. Am. Soc. Plant Physiol. West. 

 Sect., Abstr. of Papers. 



Beyer, A. (1928) Experimentelle Studien zur Blaauwschen Theorie. II. Planta, 

 5: 478-519. 



