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371 
to the sun the quantify of carbonic acid gradually diminished, which 
was evident by the water rising in the glass, at the same time a quantity 
of oxygen gradually collected and replaced the water in the upper part 
of the glass containing the mint plant. Another mint plant was placed 
in a glass containing distilled water, without having access to carbonic 
acid. A third plant was placed in a glass filled with distilled water, and 
a second glass*cdntaining oxygen was placed in the cistern as in the first 
experiment, but these two plants when exposed to light produced no 
oxygen. 
Many experiments have also been made with a view to prove that 
growth depends upon the action of leaves. If we separate two rings of 
bark with spaces without bark, and let a leaf be developed on one rino* 
and not on the other, the ring having the leaf will be found to live the 
longest. An attack of the turnip-fly or pumpkin-bug affords familiar 
■examples of the injurious effects of destroying leaves; if these insects 
attack the seminal or primary leaves, the death of the plant is generally 
the consequence, or its growth is feeble and considerably retarded. If 
we strip a plant of its leaves in summer, and prevent the growth of 
others, the branch either dies or does not sensibly increase in size. If 
the leaves of a tree with half-grown fruit are attacked by animal or vege¬ 
table parasites and destroyed, the fruit will either fall off or not ripen. 
The active vitality of leaves, and their power of forming organizable 
matter when detached and planted under suitable circumstances, has 
frequently been put to the test of late years. Andrew Knight, the late 
President of the Horticultural Society of London, planted leaves of the 
potato, which continued to live until winter. They formed at their base 
conic bodies more than two inches in circumference, which consisted of 
matter similar to that of the tuber of the potato plant. Leaves of mint 
thus treated assumed the character of the thick fleshy leaves of ever¬ 
green trees, and produced numerous roots. Not only have leaves the 
power of uniting roots, and of adding to their substance, but those of 
many kinds of plants produce buds ; and propagation by means of leaves 
is now commonly practised by nurserymen. 
The leaves of monocotyledonous plants seem to have this reproductive 
power in an eminent degree. The Rev. W. Herbert, Dean of Manches¬ 
ter, who is highly distinguished for his experiments in cross-breeding 
