100 The American Naturalist. t [Febina 
Another variation from normal respiration is known as 
insolar breathing, and which, with still some other modifica- 
tions, I need not stop to explain. To this brief statement of 
plant respiration must be added that much yet remains to be 
discovered regarding the details of the processes. 
Assimilation and respiration are the two great causes which 
disturb the relative volume of the two variable gases in plants. 
MOVEMENT OF THE SAME TWO GASES. 
_ We shall now turn to the movement of the same two gases, 
oxygen and carbon dioxide. There has never been a disposi- 
tion, as in the case of many other plant phenomena, to explain 
the movement of gases upon any other than purely physical 
principles. We have therefore to do simply with the question 
of the aids and hindrances to the establishment of an equilib- 
rium between the gases inside and outside the plant, irrespect 
ive of whether the cells are alive or dead. | 
OUTER AND INNER PRESSURE OF GASES. 
It has already been stated that the relative amounts 
oxygen and carbon dioxide inside the plamt are usually very 
different, and that within a few hours the relation of the two 
_ may be completely reversed. To this may be added that the 
pressure of the gases inside the plant is sometimes more, 80 
times less than that of the atmosphere outside the plant 
almost never the same. Hales observed ‘in his early wort 
that a mercury guage connected with the inside of the trunk 
of a tree showed an internal pressure when the hot rays © 
sun warmed the trunk. This was largely due, undoubted, 
the expansion of the gasesin the trunk, by the heat. Suc 
excess of pressure in water plants is very common, althoug 
undoubtedly many have noticed in gathering water lilies, 
other water plants. 
an On the other hand, the pressure of the gas inside the 
‘may be less than on the outside. This has long been 
