1088 BIOLOGICAL EFFECTS OF RADIATION 



depends on cell turgidity as the basis of his explanation of the results 

 obtained in numerous decapitation experiments. A portion of his dis- 

 cussion is perhaps best summarized in his own words. 



"The frequent occurrence of apical guttation is recalled, it is shown 

 that the localization of the apical hydathode explains the direction of the 

 usual autonomous curvature of the coleoptile. The permeability of the 

 coleoptile tissues are increased by light, and therefore light falling on 

 the apex increases the apical guttation. Lateral light falling on the apex 

 therefore increases the rate of flow from the vein nearer the light and thus 

 produces a phototropic curvature. When the apex is removed, guttation 

 is very free and growth much reduced until the cut surface is blocked. 

 If the apex is replaced immediately, the guttation is reduced and growth 

 is greater. If the apex is replaced on the side of the stump only, this 

 side only is blocked and curvature results. From the same standpoint 

 the different curvatures produced by the asymmetric replacement of an 

 apex or a ring of coleoptile tissue are explicable. If the replaced apex is 

 laterally illuminated the block is less complete on this side of the stump 

 and positive tropic curvature results. The various facts at present 

 elucidated by investigators as due to growth regulation by substances 

 diffusing from the apex, are passed in review from this new standpoint, 

 and appear to be consistent with it. On the other hand, the fact of the 

 upward flow of guttation, reported by nearly all experimenters with 

 decapitated coleoptiles on which tips are replaced, is not easily reconciled 

 with the idea of the downward diffusion of growth-regulating substances. 

 It is then shown that an interpretation of the phenomena of phototropic 

 curvature in the coleoptile which is consistent with the 'light-growth' 

 hypothesis of Blaauw can be applied with less difficulties and fewer 

 inconsistencies than any interpretation yet attempted on the lines of 

 stimulus and response." 



Although Priestley's point of view is very interesting and stimulating, 

 there are several points which it seemingly does not fully explain; for 

 example, as mentioned by him, the succession of positive and negative 

 curvatures is one problem. The more recent work with growth-accelerat- 

 ing substances presents other difficult problems not easily answered. 

 From the careful work of Went and his associates there can be but little 

 doubt that the growth-accelerating substances, though small in actual 

 quantity, play an important role in phototropic responses. The rapid 

 advances made in the accumulation of quantitative data during the past 

 five years lends hope that in the near future important relationships will 

 be obtained from the existing mass of what now appears at times to be 

 merely conflicting data. Furthermore, much of this information prom- 

 ises to have a direct bearing on the relationship of certain cell products 

 and life processes to radiation. 



