80 ANATOMY OF LEAVES. 



Leaves with respect to Colour. 



Leaves have not that brilliancy of colour which is seen in 

 the corolla or blossom ; but the beauty of the corolla, like most 

 other external beauty, has but a transient existence ; while the 

 less showy leaf remains fresh and verdant, after -the flower has 

 withered away. 



The substance of leaves is so constituted as to absorb all the 

 rays of light, except green ; this colour is, of all others, best 

 adapted to the extreme sensibility of our organs of sight. Thus, 

 in evident accommodation to our sense of vision, the ordinary 

 dress of nature is of the only colour upon which our eyes can 

 for any length of time rest without pain. 



But although green is almost the only colour which leaves 

 reflect, their variety of shades is almost innumerable. 



" No tree in all the grove but has its charms, 

 Though each its hue peculiar ; paler some, 

 And of a wannish grey ; the willow such, 

 And poplar, that with silver lines his leaf; 

 And ash far stretching his umbrageous arm : 

 Of deeper green the elm ; and deeper still, 

 Lord of the woods, the long surviving oak."* 



The contrast between their shades, in forests, where different 

 families of trees are grouped together, has a fine effect, when 

 observed at such a distance, as to give a view of the whole as 

 forming one mass. 



A small quantity of iron, united to oxygen in the vegetable 

 substance, and acted upon by rays of light, is said to give rise 

 to the various colours of plants. f If this theory is correct, the 

 different shades of colour in plants, must be owing to the differ- 

 ent proportion in which the iron and oxygen are combined. 



To quote the words of a celebrated Chemist, " When Nature 

 takes her pencil, iron is the colouring she uses." 



LECTURE X. 



Anatomy and Physiology of Leaves Their use in the Vege- 

 table System. Appendages to Plants. 



LEAVES are compared to the lungs of animals ; they are or- 

 gans for respiring, perspiring, and absorbing. When leaves 



* Cowper. 



t This idea coincides with the supposition, that the green colour of leaves is 

 changed to brown by the loss of an acid principle ; that when the petals of 

 flowers turn from purple to red, they have an increase of an acid. The base of 

 this acid is oxygen. 



Colour of leaves Different shades in the colour of leaves What is the causo 

 of the different shades of colour in leaves ? The use of leaves in the vegetable 

 economy. 



