984 EECORD OF CURRENT RESEARCHES RELATING TO 



ordinary evaporation from a free surface of water, but also to the 

 influence of light. Consequently, under similar conditions, a plant 

 transpires more under the action of light than in darkness. 



2. The action exerted by light on the transpiration of plants 

 augments in proportion to its intensity ; consequently under similar 

 conditions transjiiration reaches its maximum shortly after noon. 



3. Light favours transpiration only by that part which is absorbed 

 by the colouring substance of the organ ; therefore, under similar con- 

 ditions, the organ which is coloured with the greatest intensity 

 transpires more, and the transpiration is most active in the part of the 

 spectrum where the light is most absorbed. 



4. The luminous rays which are absorbed by the colouring sub- 

 stance of an organ alone favour the transpiration of the organ ; there- 

 fore, under similar conditions, the transpiration of a coloured organ 

 will attain its minimum under the influence of light of the same colour 

 as the organ, and its maximum under the influence of the comple- 

 mentary colour, 



Heliotropism.* — In a preliminary account of a recent series of 

 observations on heliotropic phenomena, J. Wiesner gives the following 

 as some of the more important points arrived at. 



In very strong light, even rays which have no heliotropic proper- 

 ties, possess under certain circumstances the property of strongly 

 retarding growth. The prevalent theory that only rays belonging to 

 the more refrangible half of the spectrum have the power of inducing 

 heliotropism and hindrance to growth must be modified, since the 

 ultra-red, red, orange, and even yellow rays, possess this jiroperty 

 under certain conditions. The liability to heliotropism is always the 

 result of the relatively greater extensibility of the cell-wall on the 

 shaded side of the organ ; but the curvature itself is only completed 

 by turgidity. Positive heliotropism and negative geotropism act in 

 opposition to one another in erect organs ; in vertical organs which 

 grow downwards the heliotropic and geotropic effects co-operate. 

 Whenever heliotropism depends on definite mechanical processes 

 which take place in the cells, it must be regarded as a phenomenon 

 of adaptation, and may commence even in organs which have not 

 themselves any reference to light in their functions. The author 

 believes the physiological purpose of heliotropism to be to place in 

 the most favourable intensity of light those flowers which are de- 

 pendent on insect-fertilization. In those plants where heliotropism 

 would be injurious, no tendency is found towards heliotropic curva- 

 ture. 



Formation of Chlorophyll in the Dark.f — M. d'Arbaumont has 

 followed out the investigations of M. Flahault | respecting the 

 apparent exceptional formation of chloroj^hyll without access of light. 

 The case to which he has paid especial attention is that of the appear- 

 ance of chlorophyll-grains in the internal tissue of the ripe fruit of the 



* ' SB. k. Akad. Wiss. Wien,' Ixxxi. (1880) p. 7. 

 t ' Bull. Soc. Bot. France,' xxvii. (18S0) p. 89. 

 J See this Journal, ante, p. 298. 



