182 Macfarlane, The Sensitive Moveim-iits of some Flowering Plants. 



surfaces parallel to, and therefore well protected from, the rays r 

 while the third had fallen through 55°, and then presented one 

 edge and parallel surfaces, to the rays. If we consider the move- 

 ment of each leaflet to be caused by changes in the pulvinus cells, 

 the pulvinus of the first leaflet having been fully exposed to the 

 intense rays, the leaflet would become strongly reflexed. The 

 pulvinus of the second leaflet would be protected after movement 

 of the leaflet through a small angle, while the rays would pass 

 parallel to the surface of the third leaflet after it had fallen to the 

 extent indicated above. ßut here and in every movement of sen- 

 sitive plants the writer considers that there is an intimate relation 

 between the pulvinus cells and the entire living tissue of each 

 leaflet, so that any change occurring in the pulvinus tissue is pro- 

 bably not confined to it alone, since vice versa his experiments 

 on Dionaea, Mimosa, Cassia, Oxalis etc. prove that Stimulation 

 of any part of a leaf or leaflet is propagated to the motile 

 centre. 



If we attempt now to answer the questions: Are the more 

 refrangible rays injurious to plant life when above an Optimum 

 intensity for each species ; and if so, why are they injurious ? 

 several publications of recent years seem to help us. 



The consensus of opinion now is, that metabolic changes go 

 on most actively under the more refrangible rays of the spectrum. 

 As proof, mention need only be made of the results obtained by 

 Wiesner, George He n slow, Purjewicz and Engel mann 

 among others. These normal processes which are often carried on 

 at rather low temperatures and in weak sunlight, are characterized 

 by the setting free of secondary produets, not a few of which. 

 are useless or even deleterious for plant growth. An increase 

 above the Optimum light intensity will be associated with in— 

 creased chemical activity, and the setting free of greater quantities 

 of decomposition products. The para heliotropic movements will 

 so act as to prevent this. The fact that exposure of non-sensitive 

 leaves on a tree to direct and intense light causes more rapid de- 

 composition of the food stuffs than on other leaves or even parts 

 of the same leaf, is well known and is apparently to be explained 

 on the same principle. 



But a light intensity greatly less than that needed to inso- 

 late any sensitive plant may affect the growth of tissue by its 

 more refrangible rays. Thus Ludwig Klein*) proved that the 

 cessation of growth during day in the conidial stalks of Botrytis 

 cinerea is solely due to the blue-violet rays and that growth as 

 well as spore-formation is promoted by red-yellow rays. Ward's 

 experiments with plate cultures of Bacillus anthracis**) prove that 

 a certain intensity of the more refrangible rays is prejudicial and 

 in most instances fatal to this fungus. Sorokin's older experi- 



*) Bot. Zeitung. 1885. p. 14. 

 **) Proc. Roy. Soc. Vol. LIII. p. 25. 



