xxii] ILLUMINATION 279 



white disc lowered into the water remains visible to a depth 

 varying between 6*8 metres in summer and 14-6 metres in 

 winter. The annual mean was found to be io*2 metres. This 

 method is a rough one, but it gives some idea of the penetrating 

 power of the luminous radiations. The results obtained har- 

 monise with the observation that chlorophyll may be developed 

 without loss of intensity by plants living at a depth of 10 metres. 

 In the Jura lakes, however, which are not very transparent, 

 some etiolation is produced even at 4 to 5 metres, in the case of 

 Naias and the submerged leaves o^ Nymphaea luted^. 



Some hydrophytes are dependent upon direct sunlight; the 

 Podostemaceae, for instance, are rarely to be found in shady 

 places where the water does not receive at least some hours 

 of sunshine during the day^. Certain water plants, on the other 

 hand, such as species of Utricularia and Ceratophyllum, perish 

 when exposed to strong illumination^; and, of submerged 

 plants in general, it is undoubtedly true that the conditions, 

 under which they live, approximate to those of 'shade plants' 

 upon land**. Their response to these conditions is also similar, 

 and they share the characteristics of delicacy of lamina, absence 

 of a well-differentiated palisade-tissue and presence of chloro- 

 phyll in the epidermis s. An attempt has been made to trace the 

 peculiarities of submerged plants to the direct etiolating action 

 of the obscurity in which they live '^, just as it has been suggested 

 that the aerating system in their tissues was originally due to the 

 direct effect of the medium". We may accept this view so far 

 as to acknowledge that the influences in question may, in both 

 cases, have played a part in the first initiation of the aberrant 

 structure of submerged plants, but such direct effects are 

 scarcely adequate to explain the structure of the most highly 

 modified forms which have lost the power to live on dry land. 



In certain water plants showing heterophylly, the intensity 

 of the light is one of the factors concerned in determining which 



1 Magnin, A. (1893}. 2 Willis, J. C. (1902). 



3Goebel, K. (1891-1893). * Schenck, H. (1885). 



5 Stohr, A. (1879). 6 Mer, t. (1880I). ^ See p. 259. 



