CHAPTER XIV. 



EXCESS OF LIGHT. 



According to the discoveries, already made in great numbers, on the 

 influence of heat on the different vegetable processes, it must be supposed, 

 from the outset, that not only does a minimal limit exist for the action of 

 light, but that also a special degree of illumination exists in each plant for 

 each process and for each combination of the vegetative factors, which can 

 be termed the optimum. The exceeding of this degree introduces a retro- 

 gression in production. In fact, the observation has already been made for 

 a number of plants that, if the light is increased above a certain amount, 

 the assimilation, perceptible in the elimination of oxygen, does not increase., 

 but remains stationary^ or indeed may decrease-. A normal carbon dioxid 

 content in the air is presupposed in this, for even when the air contains too 

 large an amount of this element, the elimination of oxygen retrogresses, as 

 has been proved already by Boussingault and, later, by Pfeffer^. An 

 optimum illumination may be seen in the appearance of the plant since this 

 loses its deeper green color, with a considerable, increase in the intensity of 

 light above the optimum ; then it assumes a yellowish color. 



That the dark green leaves of camellias show a yellowed condition, 

 when moved from the conservatory into sunny places out of doors, is well 

 known. The camellia is a Japanese plant which grov/s under trees. It is 

 content with small quantities of light and, with the strong rays of our 

 summer sun, soon loses more chlorophyll through oxidation than can be 

 formed by the process of reduction. The breaking down of the chlorophyll 

 by the taking up of oxygen (taking place also in the dark in the presence of 

 bodies which easily take oxygen from the air and form ozone, Turpentine 

 oil) is known to be connected with different groups of rays. According to 

 Wiesner, the yellow rays, and the green and orange ones on both sides of 

 them, show the greatest energ>^ in the breaking down of the chlorophyll in 

 the light. 



Another example of yellow leaves with a high intensity of light is 

 offered by some varieties of coleus with yellow variegated leaves. These 



1 Reinke, Li., Untersuchungen iiber die Elnwirkung-en des Lichtes auf die Sauer- 

 stoffausscheidung der Pflanzen. Bot. Zeit. 1883, No. 42 ff. 



2 Famintzin, Effet de I'intensite de la lumiere, etc.; cit. Bot. Centralbl. 18S0, 

 p. 1460. 



3 Pfeffer, Arbeiten d. Bot. Instituts zu Wurzburg-, ed. by Sachs, Part 1. 



