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Physical Structure of Plants. 63 
recommendation. For cpa we we addins two general laws 
which spring from this theory 
Ist: No hypotheses nor scxuments can be hand on the com- 
position of the gases expired by plants, without a rigorous regard 
to the influence of disturbing causes—as the amount of light, 
gas of the fluids of the soil, of the atmosphere, &c. 
2d. No experiments on the action of plants in einige or 
otherwise, can be adduced for physiological argumentation, unless 
made in atmospheric air. Many observations have been made on 
plants immersed in water and artificial atmospheres, which can- 
not be received, because the gases employed have penetrated the 
interior in proportions differing from those in the case of com- 
mon air. The nitrogen obtained by Saussure may ina great 
measure have been derived from the additions of carbonic acid 
made by him to the atmospheres in which he experimented ; 
and in the observations made in water, much of the mapseets is 
tinguestionably derived from the fluid. 
- 39. Finally I would present the following summary of con- 
pa rage as fairly deduced from the preceding experiments. | 
he epidermis of plants, so far as experiments have been 
fei is porous and permits the passage of gases, according to 
the physical laws of penetration. 
‘2. The roots, during the existence of chemical changes in 
plants, absorb such gases only from the soil fluids with which 
they are in contact, as will ae oe satisfy the indications of the 
internal atmosphere. 
- The internal gas ‘of plants, or plant atmosphere, is contin- 
ually fluctuating with the forces which operate upon it; during a 
State of activity in the plant, it resembles a mixture of nitrogen 
86:75, oxygen 13-25 per cent., but at night appears to contain 
more oxygen, and from 2 to 3 per cent. of carbonic acid. 
4. Its active or normal composition is that indicated by a mix- 
ture into which carbonic acid and oxygen are being diffused 
during day-light. 
5. The porosity of the entire plant is fully established by its 
action on artificial atmospheres. . 
Therefore the physical structure of plants is that of a porous 
system, subject to all the laws of diffusion, and endowed with 
no vitality other than that resulting in the formation and devel- 
opment of Cytoblasts and their arrangement after a definite type. 
New York, March, 1846, 
