Atmospheric Nutrition of Plants. 
533 
takes place only in direct sun light ; this, however, is a mistake, for tho 
reflected rays and diffused light are equally as capable as direct light 
of producing this effect. As carbonic acid is the chief source 
whence plants derive their carbon, the question naturally arises : — Is 
there sufficient in the atmosphere to meet all the requirements not 
only of wild but cultivated plants? At first sight this atmospheric 
supply would appear to bo anything but sufficient for the purpose, for 
according to De Saussure's accurate and precise determinations of 
the amount, there arc only about four parts of carbonic acid in every 
10,000 parts of air. These determinations have been confirmed by 
many other observers, and it is therefore not far wrong to say that 
every 10,000 parts of air contain from 4 to 5 parts of carbonic acid. 
This is a small relative proportion, doubtless ; but when we consider 
the enormous volume of the atmosphere, the perfection and beauty of 
the apparatus which plants are provided with for absorbing the car- 
bonic acid diffused around, and constantly wafted in all directions 
towards their leaves — when we further consider that in some instances 
plants arrive at maturity which have had no other source from whence 
to derive their carbon, wo are forced to conclude that the atmo- 
spheric supply is sufficient for most plants. This idea does not pre- 
clude another, that carbonaceous matters in the soil are most essential 
for certain purposes ; but as a rule it is the carbonic acid of the air, 
and not the carbonaceous matters in the soil, which supplies carbon to 
plants. 
Observations on the wants of plants have been somewhat enlarged 
of late years, and I believe I am not wrong in stating that whilst some 
plants entirely rely upon the carbon they find in the atmosphere, 
others are greatly benefited by carbonaceous matters, sometimes 
termed humus, which it is desirable should be present, in the soil. I 
believe that cereal crops are not dependent in any great measure upon 
any carbonaceous or organic matters in the soil ; whilst root crops, 
turnips, mangolds, and others are materially benefited by the carbon 
present in the humus. Whether it is, however, in the shape of pre- 
pared organic matter that the humus acts usefully as a source of carbon 
to roots, or whether it is because the humus in the soil is continually 
undergoing a change, and producing carbonic acid in the very soil 
which grew the roots, I leave for the present undecided. Indeed, we 
have no very distinct experiments, nor are they easily instituted, to 
show that organic matters as such arc taken up by plants ; but this is 
certainly known as an important fact, that the air in the soil itself 
contains a very much larger proportion of carbonic »cid than the 
atmosphere resting upon it. 
Some years ago M. Boussingault made some very careful experiments 
upon the amount of carbonic acid which occurred in soils, and he 
found that whilst the atmosphere resting upon the soil only gave 4 
to 5 in every 10,000 parts, the air in sandy soil recently manured, 
contained 217 parts of carbonic acid in every 10,000 parts. Shortly 
after rain the air from the same soil was again analyzed, and was 
found to contain as much as 974 parts of carbonic acid ; evidently 
showing that the wetting of organic matter, and the rapid decomposi* 
