170 LIGHT AND PROTOPLASM [Cn. VII 



justifies the conclusion that it is the rays absorbed by the plant 

 pigments which enable them to do their work in the decompo- 

 sition of carbon dioxide. The effective absorbed rays are, 

 moreover, chiefly those towards the red end of the spectrum, 

 those having over 525 x 10 12 vibrations per second (i.e. below 

 the D line).* 



In conclusion it may be said that the greater proportion of 

 the radiant energy entering the plant tissue is absorbed. Thus 

 MAYER ('93) has shown that of dark radiant heat at 100 

 about 80% is absorbed by a leaf through which it passes, and 

 this proportion is about the same whether the leaf is thick or 

 thin. Of this absorbed heat perhaps less than 10% is absorbed 

 by the chlorophyll. The rest must be used up in the vital 

 processes other than assimilation. 



b. The Chemical ^Effect of Light on Metabolism must now be 

 considered; and of this we must notice at the outset two 

 degrees. The greater effect, which is a fatal one and the better 

 known, will be treated of further on. The lesser effect is less 

 striking, yet it must be included in the greater. It shows 

 itself in a disturbance of metabolism. 



This disturbance of metabolism is evinced in some green 

 plants by heightened production of carbon dioxide and the 

 formation of chlorophyll ; and it is noteworthy that a similar 

 result occurs among Infusoria, according to the observations of 

 FATIGATI ('79), who finds the violet rays more active than the 

 green in this process. Among the Metazoa light produces im- 

 portant chemical changes in the retina of the eye, and especially 

 in the skin, facilitating the production of pigment. That im- 

 portant chemical changes take place in the illuminated retina 

 follows from the experiment of placing the electrodes at oppo- 

 site surfaces of the frog's retina. The galvanometer shows in 

 the darkened eye a slight " current of rest " flowing from the 

 front face to the deeper-lying part, containing the cones. If 

 now the retina be suddenly illuminated by blue, green, yellow, 

 red, or white light, a current, the result of chemical action, 

 appears flowing in the opposite direction ; this continues for 



* Other, less important, thermal effects of light on plants are found in the 

 formation of chlorophyll and in the quickening of transpiration, which seem 

 chiefly due to the red and ultra-red rays. 



