MOVEMENTS OF GASES IN PLANTS. 
6gi 
above with a lid in two portions, and surrounding the stem so as completely to cover 
the earth in the pot. If the soil is dry the plant withers. If a bell-glass is placed over 
it the plant revives, and again withers if it is removed. This shows that the withering 
is the result of increased, the revival the result of diminished evaporation from the 
leaves when the roots convey but very little water to the plant. If cut shoots are 
allowed to wither and are then suspended in air nearly saturated with aqueous vapour, 
the leaves and younger internodes again revive, although the whole shoot continues 
to lose weight from evaporation. This phenomenon results from the water passing 
from the older parts of the stem to the younger withered parts, as must be concluded 
from Prillieux's experiments \ 
Sect. 3. — Movements of Gases in Plants^. All growing cells of a plant, or 
all that are otherwise in a condition of vital activity, are continually absorbing atmo- 
spheric oxygen and giving back in its place a nearly equal volume of carbon dioxide. 
The cells which contain chlorophyll have in addition the property, under the influ- 
ence of sunlight, of absorbing carbon dioxide from without, exhaling at the same 
time a nearly equal volume of oxygen mixed with some nitrogen. In proportion to 
the activity of the chemical processes which take place within the cells, the movements 
of gases occasioned by them vary greatly in rapidity. The formation of carbon 
dioxide at the expense of the atmospheric oxygen takes place continuously and 
in all the cells; but the quantities concerned are small in proportion to the large 
amount of carbon dioxide which is decomposed in the green tissues, and in ex- 
change for which equal volumes of oxygen are exhaled. Some idea of the activity 
of this last-named process is obtained by reflecting that about one-half the (dry) 
weight of the plants consists of carbon which has been obtained by the decompo- 
sition of atmospheric carbon dioxide in tissues containing chlorophyll under the 
influence of light. 
Oxygen and nitrogen are permanent gases, as also is carbon dioxide within the 
limits of the temperature of vegetation, and indeed far below it. Aqueous vapour, 
on the contrary, is only produced from water within these limits, and under certain 
conditions even returns to the liquid state. In other respects aqueous vapour be- 
haves just like oxygen and nitrogen in reference to the processes to be considered 
here. 
When the gases with which we have to do are traversing closed cell-walls, 
diffusing themselves through the cell-sap, or permeating or escaping from the 
protoplasm, chlorophyll-granules, &c., their motion is a molecular one of diffusion. 
When they fill in their elastic condition the intercellular spaces, vessels, cells destitute 
of sap, or the large air-cavities among the tissues, it is a movement of the whole 
mass depending exclusively on expansive force. The movements of diffusion tend 
to bring about conditions of equilibrium which depend on the coefficient of ab- 
sorption of the gas by a particular cell-fluid, on the composition of the cell-wall, 
&c. on temperature, and on the pressure of the air. But these conditions are 
continually varying; and the equilibrium which is aimed at is being still more 
continually disturbed by chemical changes on which depend the metamorphosis 
^ Prillieux, Comptes Rendus, 1870, vol, II. p, 80, 
^ Sachs, Handbuch der Experimental-Physiologie, p. 243. — Müller, Jahrb. für wiss. Bot. 
vol. VII. p. 145. 
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