CHAP. XI. 
FOOD OF PLANTS. 
367 
to unite with the tissue of vegetation, or to enter into new 
combinations with water, atmospheric air, or other elements 
that it finds itself in contact with : whence proceed the gummy, 
amylaceous, resinous, oily, and other products peculiar to the 
vegetable kingdom. Upon this subject it has been observed 
by a modern writer, “ that, if the roots of a plant are placed 
in a close vessel, in distilled water, from which carbonic acid 
has been carefully expelled, the plant may increase a little in 
size, in consequence of the decomposition of the water, and the 
combination of its elements with the vegetable system ; but it 
is only when carbonic acid is added, that the plant acquires its 
natural vigour and rate of growth. But, if a plant is placed 
in solid carbon, and you water it with distilled water, it 
might as well be planted in powdered glass, until the carbon 
begins to combine with the oxygen of the air, and to form 
carbonic acid. Sir Humphry Davy placed a plant of Mint 
in water mixed with carbon in a state of impalpable powder, 
and he found that not a particle could enter the roots. If 
we look to the effects of manures, we shall find that in most 
cases, except when their object is to alter the state of the soil 
mechanically, or to act as stimulants, as is probably the case 
with sulphate of iron, their energy is in proportion to their 
capability of forming carbonic acid. Yeast, for instance, 
which is one of the most active manures we have, is so from 
possessing, beyond all other substances, the power of exciting 
fermentation, and thus of causing the formation of carbonic 
acid among the vegetable matter which lies buried in the 
soil. 
“ While, however, all experiments combine to prove that 
carbonic acid is the most essential of the elements upon which 
plants are nourished, it is necessary that the student should 
be aware that other species of matter are constantly taken 
into the system, and probably, therefore, contribute to their 
nutrition. 
“ Water is one of these. Although we know that a very 
large proportion of all the water absorbed by a plant is lost 
again by evaporation, yet the experiments of Theodore de 
Saussure have shown that a portion of it is actually solidified. 
He found that when plants are grown in a close vessel, in an 
