1882.] Structural Features to JS'yctitropism, Sfc. 



271 



in mechanical and physiological strength, the mechanically strong ones 

 being characterised by the nature and amount of formed materials, 

 the functionally strong ones by relative excess of protoplasm, and 

 specially of chlorophyll corpuscles. Every palvinus, in fact, consists 

 of different portions, in some of which functional, and in others 

 mechanical power is present in excess*, and, as from their localisation 

 the functionally and mechanically strong portions are opposed to one 

 another, the position of the leaf as determined by the pulvinus must 

 vary with the relative strength of these. But it can be further shown 

 that the mechanically stronger portions constantly tend to cause the 

 leaf to assume the nocturnal position, while the physiologically strong- 

 ones act in the reverse direction. Hence, there must be constantly 

 recurring fluctuations in the relative strength of the different areas, 

 corresponding with the recurring variations in position of the leaves. 

 But these variations in relative strength must be ascribed rather to 

 changes occurring in the physiologically strong than in the mechani- 

 cally strong portions of tissue, to elevations and depressions of strength 

 connected with functional rather than structural conditions. The 

 functionally stronger areas, accordingly, must be regarded as con- 

 stantly becoming relatively stronger diurnally and weaker no cturnally, 

 and this recurring fluctuation of strength may be fairly ascribed to 

 variations in tension, due to variations in the absorption of liquids 

 connected with the specific diurnal functional activity of the proto- 

 plasmic elements of the tissues, and specially of the chlorophyll 

 corpuscles under the influence of light. 



When the varying kinds and degrees of nyctitropic movements 

 exhibited by the leaves of the individual species at various stages of 

 their development are compared with the coexisting structural features, 

 additional evidence is found of the essential dependence of the former 

 on the latter. The positions of the leaves in the bud and at later 

 stages, and the assumption and persistence of special movements, are 

 clearly related to the varying degrees of development and the condi- 

 tion of particular portions of the contractile organs. 



The phenomenon of reverse movement towards the diurnal position 

 during the latter portion of the night, and independent of any renewed 

 incidence of light is not necessarily opposed to a belief in inequalities 

 of functional activity and tension of particular areas of the pulvinar 

 tissues under the influence of light as the essential cause of nyctitro- 

 pic movements. The nocturnal reversion may be ascribed to another 

 agency than a rise in functional distension independent of light. It 

 may arise from alterations in relative tension of the different portions 

 •of tissue due to differences in structure. Those areas in which diurnal 

 functional activity and the incident increased absorption and tension 

 must attain a maximum are also those in which the greatest facilities 

 for the redistribution of fluids are provided by the nature of the 



