47 



period, while in others it takes place many months afterwards. 

 The red colour is generally produced by the perforations of 

 insects and by the development of Entophyta, where the leaf 

 merely suffers an interruption in its normal development, but 

 which is not accompanied with death. 



M. Mohl, on the other hand, points to a resemblance which 

 appears to exist between the production of the red colour in 

 leaves and that in fruits ; this might perhaps be compared 

 with the process of ripening of the juicy envelope of fruits. 

 M. M ohl supposes that the formation of red colour in fruits is 

 independent of nutrition, as fruits even ripen when separated 

 from the plant in an immature state : much however might 

 be adduced against the adoption of this view, and I cannot 

 concur in it. 



In leaves, it is the cold, observes M. Mohl, which interrupts 

 their vegetative process, and causes, when light acts on the 

 leaves, red colour to be formed in them. In pericarps, the de- 

 velopment of which is a more complicated process, it is the 

 warmth which favours the perfect development, arid thus 

 hastens the last period of life. As the result of these observa- 

 tions it may be admitted that the formation of colours accom- 

 panies various interruptions of the normal process of vegeta- 

 tion of the leaves and of their elaboration of the crude sap ; 

 on the other hand it must be considered as merely accidental 

 that it occurs in those cases in organs near death, since it hap- 

 pens, as I have stated above, frequently in leaves which con- 

 tinue to live for some time and again become green. 



If leaves which have become red in winter be examined, we 

 find the chlorophylle little or not at all changed, but perceive 

 together with this a red colour in the cells, which, for the 

 most part, is situated in the cells of the epidermis. I have 

 however frequently observed red -coloured cellular sap con- 

 taining green-coloured globules, even in the green cortical 

 layer. It is more seldom that red cellular sap is developed 

 only in the outer layers of the diachyma of leaves ; in this case 

 also, says M. Mohl, it is found in leaves which recommence 

 their vegetation in spring more confined to the outer cells, as 

 in Chelidonium majus, Hedera Helix, &c., while in leaves which 

 die off in spring the formation of colour affects nearly all the 

 cells of the diachyma, as in Isatis tinctoria. 



