153 
GARDENING AS A SCIENCE. 
] VEGETABLE PHYSIOLOGY.— No. VII. 
The Functions of the Leaves now require attention. A concise view of 
he situation and structure of the vital organs has been taken in Article 4, 
oage 81, of the present volume ; the reader is, therefore, in a degree prepared to 
sonsider the hypotheses which have been or are advocated, by leading authorities, 
if the offices which they perform in the vegetable economy. 
In general terms it may, without hesitation, be asserted, that the leaves are 
imong the most important of the organs, inasmuch as they perform the functions 
if respiration , perspiration , and elaboration of the nutritive and specific fluids of 
he plant. The structure of the leaves leads to a shrewd suspicion of their vast 
onsequence, and when we have shown that it comprises a system of cells and 
tomates , that is, orifices or mouths (from stoma — Greek — a mouth), it becomes 
lmost self-evident, as some have asserted, that each leaf, and division of a leaf, is 
, species of lung, or lobe, by and through which the vital secretions are prepared 
nd conducted. 
In the first instance we shall allude to the substances produced in the leaves ; 
nd, in describing these cursorily, we prefer an appeal to the authority of the 
hemists, Davy and Liebig, because, by the analysis which they have instituted, 
icts will be rendered quite evident, which will prove that, by the combination of the 
lements of the atmosphere alone, all the organic processes of vegetable maturity 
an be effected, thus leaving little to be performed by the roots beyond the intro- 
asception of a watery solution of a few inorganic metals and alkaline salts. 
Sir Humphry Davy’s arrangement of the compounds found in plants is made 
i the following order: — 1, Gum, or mucilage, and its different modifications; 
jjj Starch; 3, Sugar; 4, Albumen; 5, Gluten; 6, Gum Elastic; 7, Extract; 
j Tannin ; 9, Indigo ; 10, Colouring Principles ; 11, Bitter Principles ; 12, Wax ; 
3, Resins ; 14, Camphor; 15, Fixed Oils; 16, Volatile Oils; 17, Woody Fibre ; 
8, Acids. 
The properties, or rather the chemical products by analysis, of a few of the 
bove substances will be described according to the estimate of Davy : they will 
iow how simple, and yet how infinitely diversified are the instruments of nature, 
'n one point we must previously caution the student. It is customary to say — 
ad too many take the assertion for granted — that a certain product consists , or is 
imposed of such and such elements. But this language is not correct, for it 
Isifies the judgment : thus, in citing (for example) the first of Davy’s list — -Gum 
-we read that — 
“ From the analysis of MM. Gay-Lussac and Thenard it appears that Gum Arabic 
contains in 100 parts, of Oxygen and Hydrogen in the proportions necessary 
to form water . . . . . . . . . . . . 57*77 
<l And of Carbon 42*23 
VOL. XI. NO. CXXVII. 
X 
100 ” 
