' Leaves in Autumn. 271 



as to demand a separate examination, and although in general 

 'it cannot be, denied that the fall of the leaves is often preceded 

 by their change of colour, there are many cases in which the 

 leaves fall green, and others in which they change colour before 

 falling. This distinction is of some importance, for if the change 

 of colour in the leaf be only a casual antecedent of its fall, it 

 must be considered as a commencement of death, as most phy- 

 siologists have considered it ; whereas I think it ought to be 

 Viewed as a phenomenon of the life of the vegetable, . a conse- 

 quence or continuance of the regular action of the same agents 

 which preside over the other functions of the plant,*— an opinion 

 which the few facts adduced in this memoir may perhaps serve 

 to confirm. 



It is, as every body knows, at the end of summer, or in the 

 course of autumn, that the change of colour in the leaves, which 

 forms the subject of our examination, is produced. However 

 varied the tints may be which they present, it may be said, that, 

 with a small number of exceptions, they come to shades of yel- 

 low or red, which are at this period the predominant colours in 

 the landscape. This change is far from being sudden. In general 

 the green colour disappears by degrees in the leaf ; many leaves, 

 however, as those of the acacia and apricot, begin to grow yel- 

 low here and there, and in spots. In others, as the pear-tree, 

 &c. there long remain dots of a beautiful green on the orange or 

 yellow-ground of the leaves. Some, as those of the Rhus coria- 

 ria, begin to change at their edges, and especially at the tip. 

 The nerves and the parts of the parenchyma which touch them, 

 seem to retain the green colour longest. I think I have ob- 

 served that the leaves whose green is deep assume the red co- 

 lour, and those whose green is pale, the yellow or yellowish 

 tint. Most of the leaves, however, which become red, pass 

 through the yellow as an intermediate tint ; as in the Rhus co- 

 riaria. 



Influence of Light. — It were easy to sec that the action of 

 light exercises a great influence upon the autumnal change of 

 the colour of the leaves in the leaves which naturally cover each 

 other in part, the uncovered portion being always more quickly 

 and more deeply coloured. It was of importance to determine 

 if the phenomenon miglit take place in darkness, and on entire- 



T % 



