23 <5 PLANT PHYSIOLOGY 



Which o] tJie rays in white light induce phototropic curvatures? 



EXPERIMENT. Prepare 4 dark chambers, each some 15 cm. 

 square; in the middle of one side of each of them, leave an opening 

 4 cm. in diameter, and cover this outside with a Soyka flask, which 

 is attached so that no leakage of light can occur (by black gummed 

 paper); fill the flasks with, respectively, red, green, blue, and white 

 (control) liquids made up spectroscopically pure according to the 

 directions earlier given (page in). Place in each chamber a small 

 pot containing Oats or Barley, germinated in darkness to about 2 cm. 

 high ; give them all equal exposures, and observe at intervals to ascer- 

 tain under which the phototropic response is quickest and most pro- 

 nounced. 



The ideal way to solve this problem is through use of a spectrum thrown 

 laterally upon the young seedlings, but the practical experimental difficulties 

 are very great. 



Special phases of phototropic irritability, susceptible of ex- 

 perimental study, are the movements of chlorophyl grains, and 

 the locomotive movements of some free-swimming organisms 

 (phototaxis). 



All of the responses to light so far considered are to light 

 from one direction, but there are cases of another character, of 

 which the sleep movements of Clover, Oxalis, or Mimosa leaflets 

 are typical, in which the stimulus is not unilateral, but diffused, 

 and the responses are said to be photonastic instead of phototropic. 

 These sleep movements, indeed, are of such frequency that 

 the sleep response has been given a special name, nyctitropism 

 (though it should be nyetinasty), and they are readily accessible 

 to experiment. 



SUGGESTED EXPERIMENT. Take at midday a potted Oxalis or Mimosa 

 with expanded leaflets, and enclose it in a ventilated dark chamber; then 

 note any change in the leaflets, and the rapidity of the effect. Also ascer- 

 tain whether there is any difference in the response at different hours of the 

 day. 



The foregoing experiments will give the student an idea of 

 the essential facts of phototropism, and he should now bind and 

 weld his personally acquired knowledge into a broader and 

 deeper comprehension by study of the literature. 



