266 LECTURE XIII. 



In the case of persistent leaves, the vitality is also diminishing, 

 but the diminution is due rather to the unfavourable external 

 conditions, especially the low temperature, which prevail in 

 autumn. It has been already mentioned (p. 239) that a certain 

 temperature is necessary to the formation of chlorophyll, and 

 if this condition is not complied with, as is usually the case in 

 autumn and winter, no chlorophyll will be formed and the 

 leaves will become and remain yellow. When the tempera- 

 ture rises in the spring the yellow leaves become green again, 

 as von Mohl first observed, that is, the formation of chloro- 

 phyll is resumed. This may be effected even in the winter 

 by placing the plants in a warm atmosphere; Askenasy found, 

 for example, that when a branch of Thuja with yellow leaves 

 was kept in a warm place they slowly became green. 



It appears that the conversion of chlorophyll into the 

 yellow colouring-matter xanthophyll or phylloxanthin (see p. 

 241) is effected by a process of oxidation. Senebier observed 

 long ago that solutions of chlorophyll when exposed to light 

 lose their green colour, and Gerland and N. J. C. M tiller, in 

 repeating this observation, have found that the presence of 

 oxygen is essential to the process. The influence of light in 

 promoting the oxidation of the chlorophyll is well illustrated 

 by Pringsheim's observation that when chlorophyll-corpuscles 

 are exposed to intense light they completely lose their 

 colour in a few minutes provided that oxygen is present. It 

 is doubtless to the continuous oxidation of chlorophyll in the 

 plant that the constant presence of xanthophyll in solutions 

 of chlorophyll is due. Sachs and Wiesner have further ascer- 

 tained that the decolouration of a solution of chlorophyll goes 

 on much more rapidly in intense than in feeble light, and that 

 the rays of low refrangibility are more active in producing it 

 than those of high refrangibility. 



In many cases leaves assume other colours than yellow in the autumn. 

 Very commonly they become red. This is due to the presence of a red 

 colouring-matter (erythrophyll) or probably of a mixture of colouring- 

 matters, in solution in the cell-sap. The formation of erythrophyll 

 appears to be dependent to some extent upon light, but the exact condi- 

 tions have not been fully investigated (see von Mohl, Treviranus, Wiesner, 



