erg at SS ap 
_ per part a, fresh water 
Ee a ee eee 
és i aia 
a an nd f y 
_ Reb ao 
; Paris ae ie 
Physical Structure of Plants. 55 
sap is likewise subject to the variations of light and darkness, and 
of chemical conditions. We cannot therefore reach any certainty 
by mere argumentation as to the action of roots on gaseous mix- 
tures dissolved in fluids, and must resort to experiment for a solu- 
tion of the problem. Our researches must also be directed to a 
given point, as shown in section 11, or we obtain no trustworthy 
conclusion. x3 
19. Experiments.—On the 25th of June, 1844, I commenced 
a series of observations to determine the action of the uninjured 
roots of Datura and blue grass on the gases of common pump- 
water. The plants were obtained as directed in section 12, so 
that the roots were perfect. . Hach specimen when washed free 
from soil was introduced © > 
into a vessel of glass, of 
the figure represented in 
the engraving, and pre- ff 
viously filled with pump- {| 
ance the gases evolved 
by: the: Toots, if any, / 
would rise into the up- (S$ 
could be added to compensate for evaporation by the leaves, and 
the atmosphere be completely shut out. The plants of Da- 
tura Were nine inches high, the grass plants about five inches. 
three sets of plants were used, two specimens of each being 
Placed in darkness, A; in diffused light with the glass uncovered, 
7; and thirdly with the leaves exposed to light but the glass 
vessel covered, the roots being in darkness, C. = tae 
; 20. On the evening of the 25th, the two plants of Datura, B, 
Were placed in the study window so as to receive the morning 
light ; at 11 o'clock the next morning there was sufficient gas in 
both vessels for analysis, ($ 13,) the composition of both was 
- iMentical—nitrogen 96-9, oxygen 3-4, no carbonic acid. A test 
experiment with air gave 20-9 oxygen per cent. These two 
Plants being uninjured and exceedingly vigorous were returned to 
the receiver, supplied with fresh water, and placed in a dark cup- 
board at 10 », w. of the 26th, where they remained for thirty-six 
outs without evolving a bubble of gas. At 10 a.m. of the 
they were again exposed to light, and yielded at 2 r. m. suf- 
Bigs kta to 
i aoe 
a 
