256 SMITH : 



bring forward arguments from its distribution and the 

 conditions of its occurrence, in favour of some particular 

 advantage being the one really obtained. Thus, the history 

 of the question of anthocyan can be conveniently summarized 

 by dividing the researches into those which support the 

 " screen theory," i.e., the theory that the anthocyan absorbs 

 the rays which are destructive to chlorophyll, and those which 

 support the theory that the anthocyan by raising the tempera- 

 ture of the leaf favours transpiration and also in colder climates 

 translocation. Overton's (22) researches stand apart from 

 the rest in that they show the red colour to be a product 

 formed under certain conditions by chemical processes going 

 on in the leaf whenever those conditions are realized. Thus, 

 it is not necessary, Overton's (22) results being known, to 

 suppose the anthocyan to be an advantageous adaptation at 

 all in itself. It is only necessary to suppose that its dis- 

 advantages, if they exist, are not so great as to imperil the life 

 of the species in the struggle for existence. The foundation of 

 the screen theory was laid by Pringsheim (24), 1880. He 

 performed experiments showing the destruction of chlorophyll 

 by intense light in Nitella, Elodea, &c., and showed also that 

 under a red screen, a solution of iodine in carbon bisulphide,- 

 this destruction did not occur. His general conclusion from 

 the whole of his experiments was the theory that the pro- 

 toplasm does the actual work of carbon assimilation and 

 that the function of the chlorophyll is to act as a screen to 

 prevent oxidation of the protoplasm in too strong a light. This 

 theory is, of course, now quite discarded. Nevertheless, the 

 particular experiments here quoted are quite possibly a good 

 foundation for the screen theory as applied to anthocyan, 

 and have been so used by later writers, e.g., Ewart (7). 



Reinke (25), 1883, expressed conclusions in opposition to 

 Pringsheim's results. For us his most important result is 

 that exposing chlorophyll and other colouring matters to 

 concentrated light, with heat rays cut off by passing through 

 alum solution, he obtained no bleaching in leaves of Elodea 

 and Impatiens or in petals of Papaver and Rosa until the light 

 intensity became 800 or 1,000 times the usual intensity of 

 sunlight. These experiments are directly opposed to those of 



