178 Macfarlane, The Sensitive Movements of some Flowerin» Plants. 



under green, and that under blue screens the leaflets remain 

 open as in ordinary day light. But expansion under the red and 

 yellow screens soon takes place, the rapidity of expansion varying 

 according to the brightness of the light and the species experi- 

 mented on. An imperfect knowledge of this subsequent change 

 has caused Vines*) to give Sachs' Statement a fallacious Inter- 

 pretation, for he says „with regard to the particular rays of the 

 spectrum which determine these movements it appears from the 

 observations of Sachs and Bert that the highly refrangible rays 

 are those which are specially concerned. When a plant with its 

 leaves fully expanded is exposed to yellow light, the leaves soon 

 close, whereas if under the same circumstances the plant be ex- 

 posed to blue light, the leaves will remain expanded. Yellow 

 light acts like darkness, blue light like day-light." This condition 

 is a short-lived one, lasting at longest in the species experimented 

 with for about eight hours. 



In time then all of the screened plants had their leaves fully 

 expanded and presented this position during the morning (from 

 5.30 tili 8 or 8.30 on the average,), and before sunset (from 4 to 

 5.30 on the average) of each day. It will readily be noted, from 

 a comparison of the species stud'.ed, that the sunset variations are 

 considerable. Thus the Cassias show nyctitropism sooner than does 

 Oxalis striata . 



If the light be diffuse and thus of moderate intensity the flat 

 morning position of the leaves is retained throughout the entire 

 day, or part of it if the sun ultimately shines out, as happened on 

 September 13 th in the above records. If the light become more 

 intense no alteration, or it may be slight deflection in Cassia or 

 inflection in Oxalis occurs to leaflets of plants under the red and 

 yellow screens. When plants are under a green screen and exposed 

 to intense illumination the leaflets either remain flat or assume a 

 more or less paraheliotropic position, the angular change at times 

 amounting to 25°. In all cases under blue screens the leaflets 

 become paraheliotropic more or less powerfully, the amount of 

 angular movement being proportioned to the intensity of light. It 

 is impossible at present to say whether the blue or violet rays are 

 the more powerful. 



In all cases normal nyctitropic movement is accelerated a half 

 to one and a half hours under a red screen. But the movements 

 of the leaves and leaflets then are very peculiar. In Cassia nicti- 

 tans and C. chamaecrista some of the leaves fall, others slightly 

 rise, and in closing of the leaflets a pair, or even one here and 

 there, may shou inflexion when others onf the same leafhave not 

 moved. 



Under a yellow screen nyctitropism is not quite so accelerated 

 as under red, but the closing movements are nearly or quite 

 regulär in sequence, and in Cassias are first visible at the leat 



*) Physiology of Plants, p. 539. 



