153 
GARDENING AS A SCIENCE. 
VEGETABLE PHYSIOLOGY._No. VII. 
The Functions of the Leaves now require attention. A concise view of 
the situation and structure of the vital organs has been taken in Article 4, 
page 81. of the present volume ; the reader is, therefore, in a degree prepared to 
consider the hypotheses which have been or are advocated, by leading authorities, 
of the offices which they perform in the vegetable economy. 
In general terms it may, without hesitation, be asserted, that the leaves are 
among the most important of the organs, inasmuch as they perform the functions 
of respiration, perspiration^ and elaboration of the nutritive and specific fluids of 
the plant. The structure of the leaves leads to a shrewd suspicion of their vast 
consequence, and when we have shown that it comprises a system of cells and 
stomafes, that is, orifices or mouths (from stoma — Greek — a mouth), it becomes 
almost self-evident, as some have asserted, that each leaf, and division of a leaf, is 
a species of lung, or lobe, by and through which the vital secretions are prepared 
and conducted. 
In the first instance we shall allude to the substances produced in the leaves ; 
and, in describing these cursorily, we prefer an appeal to the authority of the 
chemists, Davy and Liebig, because, by the analysis which they have instituted, 
facts will be rendered quite evident, wliich will prove that, by the combination of the 
elements of the atmosphere alone, all the organic processes of vegetable maturity 
can be efiected, thus leaving little to be performed by the roots beyond the intro- 
susception 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 
in the following order: — 1, Gum, or mucilage, and its different modifications; 
2, Starch ; 3, Sugar ; 4, Albumen ; 5, Gluten ; 6, Gum Elastic ; 7, Extract ; 
8, Tannin ; 9, Indigo ; 10, Colouring Principles ; 11, Bitter Principles ; 12, Wax ; 
13, Eesins ; 14, Camphor; 15, Fixed Oils; 16, Volatile Oils ; 17, Woody Fibre ; 
18, Acids. 
The properties, or rather the chemical products by analysis, of a few of the 
above substances will be described according to the estimate of Davy ; they will 
show how simple, and yet how infinitely diversified are the instruments of nature. 
On one point we must previously caution the student. It is customary to say — 
and too many take the assertion for granted — that a certain product consists, or is 
composed of such and such elements. But this language is not correct, for it 
falsifies 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 %vater . . . . . . . , . . . . hl'll 
"And of Carbon 42 -23 
VOL. XI.— NO. CXXVII. 
X 
100 " 
