Light direction, phototropism

Phototropism is defined as the directed growth or bending of fixed organisms in response to light. Phototaxis is light-induced locomotion of... 

Fig. 3. Comparison of different blue-light action spectra for directional responses (2) positive phototopotaxis of Euglena4 (2) Phototropism of Philobolus121, (3) phototropism of Phycomyces40 (4) second positive curvature of the avena coleoptile58), (5) first positive curvature of the avena coleotpile39)...

Another way to measure action spectra is to employ a balance or null method, in which the criterion response is zero, but there is no need to work at absolute threshold levels. For example, two Ughts, one a fixed reference light and the other a variable test Hght, could be applied, as appropriate, from opposite directions or in temporal alternation, in such a way that no net response occurs. Examples for action spectroscopy on Phycomyces are balance experiments on phototropism and null experiments on the light-growth response. 

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