Physical Structure of Plants. 51 
furnished an epidermis that leaked so fast when the tube was 
raised from the mercury, that no experiments could be made with 
them, other specimens sustained for thirty-six hours a pressure of 
four inches and then became so dry as to crack. 
7. August 12th.—A tube similar to the preceding, but writle an 
aperture of one fourth of an inch and graduated, was closed 
by a piece of the epidermis from the under side of a cabbage leaf. 
It was filled with mercury, placed over the trough and 400 
measures of a mixed gas, containing nitrogen 87, oxygen 13 per 
cent. introduced ; there still remained two and a half inches of 
mereury. Over this was inverted a jar containing common air 
with 10 per cent. carbonic acid. The arrangement was comple- 
ted at 2 p.m, temperature 83° Fah., and the mercurial column 
descended until 11 o’clock p.m. when the tube contained 433 
measures, this remained stationary until 5 p. mM. the next day, 
femperature 81° Fahr. The internal gas yielded-on analysis, 
nitrogen 76, oxygen 17, carbonic acid 7 per cent. Therefore 
nitrogen had been evolved from the tube and carbonic acid and 
oxygen absorbed. The external mixture was not examined, as 
object in view was merely to ascertain the mechanical pene- 
tration. The foregoing plants were also examined in this way 
and all gave similar results. 
_8. Variations were made in these two experiments to attain cer- 
tainty, and the results were uniform when the same membrane 
Was used, . There was considerable difference in the rapidity 
with which gases penetrated the different specimens of epidermis. 
‘This fact is no doubt connected with the number and size of the 
stomata; and in the case of the balsam it was manifest, the epi- 
dermis being studded with them: but as this point did not spe- 
iad occupy my attention I dismiss it without further remark. 
NHS My The Nature of the internal. Gas of Plants. 
9. It having~been shown that the bounding membrane is 
Porous, the gases of the interior of the plant should apparently 
resemble the common atmosphere. But this is true only under 
one condition, that all chemical changes in the interior shall have 
been arrested for some time. It is obviously untrue in every case 
where organic movements are occurring, whereby some gases are 
enter =—S nen ae and ois we know to be the 
