70 LIGHT, VEGETATION AND CHLOROPHYLL 



several generations under glass screens which are an insuper- 

 able obstacle to ultra-violet. 



Absorption by the Epidermis 



The insensibility of plants to this ultra-violet spectral 

 region of wave-lengths longer than 2,890 A may have a very 

 simple explanation. We know that a radiation is active only 

 to the extent that it can be absorbed; a body which reflects 

 it, or is transparent to it, cannot suffer any effect from it. 

 How do the vegetable tissues behave, from this point of view, 

 in the presence of ultra-violet? As the photographic plate 

 provides a sensitive and easily used means of detecting it, 

 many investigations have been made on the subject. 



But although the presence of these rays can be revealed 

 by photography, quantitative measurements require an 

 extremely precise technique. Consequently, the various 

 experimenters have been able to give only qualitative infor- 

 mation, but they all agree in stressing the opacity of the 

 epidermis and of the cell walls. The observations are made 

 with a special microscope for ultra-violet. 



As a result, it appears that the ultra-violet cannot pene- 

 trate to the interior of the plant, like the visible and the near 

 infra-red, which are even capable of traversing the entire 

 thickness of a leaf. Obviously, therefore, it is less important 

 and its action, if it has any, is confined to the most superficial 

 and directly exposed cells. 



It would be interesting to know whether this ultra-violet, 

 which is not transmitted, is reflected or absorbed. The 

 appearance of photographs of vegetation taken in ultra- 

 violet radiation lead one to beheve that the reflection factor 

 is small. This observation confirms the intuitive impression 

 of the physicist accustomed to encounter infinitely more 

 substances capable of absorbing ultra-violet than of reflecting 

 it. In conclusion, and in the absence of direct investigation 

 and clear results, we may say that in all probability the greater 

 part of the ultra-violet radiation which strikes the surfaces 

 of plants is absorbed by the superficial layers, that the reflected 



