: i ZZM 
^m 
§ 
THE SCIENTIFIC IIISTORY OF A PLANT. 
would be totally inadequate to give us but a very small portion of that carbon which is found in 
vegetation. But another calculation was made as to a different source for the carbonic acid ; a ealcu- 
Carbonic acid supplied lation which was based upon De Saussure's accurate determination of the amount of 
by the atmosphere. carbonic acid present in the atmosphere, an amount of a little more than one- 
thousandth of its weight ; yet we find that the air contains a no less sum than 3085 billions of pounds 
of carbon, a quantity surpassing in weight not only the carbon of existing vegetation, but also that 
which is at present locked up in the mineral coal which is distributed over certain parts of the earth's 
surface. 
It may now well be asked, How lives the young plant before it comes in contact with the atmo- 
Chcmicai action of a sphere, the source of its carbonic acid ? The reply to this question is the key to the 
young plant. proper action of humus. This humus is especially useful for the support of young 
plants ; it takes oxygen from the air, and then furnishes the plant with carbonic acid — this is its 
great use. We see, then, that the process of nourishment in a young plant totally differs from those 
actions by which the well developed vegetable is supported. A young plant causes a certain quantity 
of oxygen to be abstracted from the atmosphere, while an old one furnishes us with a never-failing 
source of this gas. 
I cannot help referring to the fact, that many juicy and milky plants, in warm countries, flourish on 
Certain shrubs grow a soil destitute of humus, containing absolutely not a trace of carbon ; and sometimes 
on arid places. ^.g f ounc J being held by one point of attachment to this barren soil — such shrubs as 
the Cactus and the Caoutchouc are among this number. Baron Humboldt especially mentions the 
Cactus tribe. In his beautiful paper on the " Physiognomy of Plants," he states : — " In the waterless 
plains of South America, animals suffering from thirst seek the Melon-Cactus, a spherical plant half- 
Humhoidt on the Cac- bm'ied in the dry sand, and encased in formidable prickles, but of which the interior 
abounds in refreshing juice. The stems of the columnar Cactus often rise to the 
height of from thirty to thirty-two feet ; they are often covered with lichens, and dividing into cande- 
Aspccta of Nature, vol. labra-like handles, resemble in physiognomy some of the Euphorbias of Africa." 
ii., pp. 24 and 149. Again, in note 20 : — " When one has been accustomed to see Cactuses only in our 
hot-houses, one is astonished at the degree of density and hardness which the ligneous fibres attain in 
old Cactus stems. The Indians know the Cactus wood is incorruptible and excellent for bars and the 
thresholds of doors. There is hardly anything in vegetable physiognomy that makes so irregular and 
ineffaceable an impression upon a newly arrived person, as the sight of an arid plain thickly covered, 
like those near Cumana, New Barcelona, and Cora, with columnar and candelabra-like divided Cactus 
stems." 
We must all allow, that this fact of the growth of highly ligneous plants containing juices in 
Facta opposed to the al 'i<l plains, is not in favour of an hypothesis that considers vegetable mould as the 
old theory of humus. truc source f carbon for plants. 
Besides the property of decomposing carbonic acid, vegetables have also the power of decomposing 
Decomposition of water ivalcf : hence the source of hydrogen. At first sight we must imagine that there 
*> |,I " |K must be a marvellous energy in the chemical process of vegetation when able to 
effect what the electricity of a powerful thunderstorm accomplishes only feebly and imperfectly : 
but when we reflect upon the various methods by wliich water can be decomposed, this feeling is some- 
what limited. The metals — some at common temperature, others at a red heat, and 
the same, or more of them in contact with a strong acid ; and, as it has been 
beautifully shown by Mr. Grove, by heat alone. 
We know that this action must take place, from the fact that caoutchouc, wax, and oils, contain 
more hydrogen than oxygen; and we also know that water must be the only source of the hydrogen, 
Action of hydrogen The water is decomposed, the hydrogen is taken up into a plant with the green prin. 
upon puma. eiple of the leaf, which diminishes in quantity when oxygen is absorbed. Plants 
containing water and carbonic acid and evolving only a little oxygen, give an acid, — evolving more 
oxygen, they form a neutral substance, — evolving a large amount of oxygen they give us an oil. 
Again, chemical analysis pointed out nitrogen as a constituent of plants, and for a long time it was 
Nitrogen a constituent a question how this nitrogen was obtained; later experiments, however, have 
of piano given to them shown that it has its origin in the ammonia which is always found in the atmos- 
in ainiii'iiuii. , 
phere. 
It was found that plants would grow in charcoal, or in calcined earth containing not a trace of carbon, 
it' watered with rain water, and this because rain water contains more ammonia— hence its softness. 
So there arc two forms in which this ammonia, so requisite for vegetation, may he found: as a gas 
existing in the atmosphere (though this is seldom the case), and held in solution by water which 
/' ~~ "^S 
\ Mllll.l! ii.ll Of 
dtogon. 
hy- 
