ANATOMY OF LEAVES. 



59 



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

 Though each its hue pecuUar ; paler some, 

 And of a wannish gray ; 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, wliere difTerent fam- 

 ilies 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 sub- 

 stance, and acted upon by ra3's of hght, 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 different proportion 

 in which the iron and oxygen are combined. 



To.qmte the words of a celebrated chemist "When Nature takes 

 her pencil, iron is the colouring she uses." 



LECTURE X. 



ANATOMY AND PPIYSIOLOGY OF LEAVES — THEIR USE IN THE VEGETABLE SYS- 

 TEM — APPENDAGES TO PLANTS. 



Leaves are compared to the lungs of animals; they are organs for 

 respiring^ perspiring, and absorbing. When leaves are wanting, as 

 in the Prickly Pear, (Cactus,) the green surface of the stem appears 

 to perform their office. If you will observe a dead leaf which has for 

 some time been exposed to the action of the atmosphere, you may 

 see its skeleton, or frame-work ; this consists of various fibres, mi- 

 nutely subdivided, which originate from the petiole. This skeleton of 

 the leaf may be examined to advantage, after boiling the leaves 

 slightly, or rubbing them in water ; the cuticle, or skin, easily separ- 

 ates, and the pulp, or cellular texture, may then be washed out from 

 between the meshes of the veined net- work : thus, the most minute 

 cords of the different vessels become perceptible, with their various 

 divisions and subdivisions; these form what is called the vascular 

 system. (See Fig. 6L) 



Though in external 

 appearance, the or- 

 gans which compose 

 the vascular system of 

 plants, are analogous 

 to the bones which 

 constitute the founda- 

 tion of the animal sys- 

 tem, yet they are ra- 

 ther considered as 

 performing the office 

 of veins and arteries. 

 They are found to be 



* Cqwper. 



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 the petals of flowers change from pur- 

 ple to red by an increase of acid. The base of this acid is oxygen. 



What is the cause of these different shades of colourl— the use of leaves in the veg^' 

 etable economy— Skeleton of the leaf— Vascular system. 



Fig. 61. 



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