MOVEMENTS OF GASES IN PLANTS. 
^93 
the chemical processes that go on in the surrounding tissues. A submerged water-plant, 
for example, which contains chlorophyll, absorbs carbon dioxide from without under the 
influence of sunlight ; and at least a portion of the disengaged oxygen collects in the 
cavities. When it becomes dark this process ceases; the collected oxygen is now 
absorbed by the fluids of the tissue and gradually transformed into carbon dioxide, 
which can again diff'use back into the cavities, but partially also through the layers 
of tissue into the surrounding water. This, as well as the different coefficients of 
diffusion of the gases, causes the air contained in the cavities to have an altogether 
diff'erent composition from that in solution in the surrounding water, and this composition 
to be subject to continual change. But it is not only the chemical composition of the 
gas in the cavities that is altered in this way; the pressure is also subject to variation. 
When the oxygen which is liberated from the green tissues collects rapidly in the cavities 
under the influence of bright light, the gas is then subject to high pressure, and escapes 
with force, injuring the surrounding layers of tissue. The greater rapidity of diff'usion 
of carbon dioxide, and its slower production in the tissue in darkness, do not, on the 
other hand, allow an increase of tension of the gas to arise easily in the cavities of the 
plant when kept in the dark. 
The nitrogen of the atmosphere takes a more subordinate and secondary part in all 
these processes. It is indeed never absent from the air contained in the cavities, but is 
generally present in large quantities in it, together with oxygen and carbon dioxide. It 
is not however subject to such rapid and considerable variations, being neither used up 
nor disengaged in the changes connected with the assimilation of food in the tissues. 
(c) Land-plants differ from water-plants in that their internal cavities, when present \ 
communicate directly with the atmosphere through the stomata. The anatomical con- 
ditions show at once that these organs are only the channels of exit from the intercellular 
spaces which are in connection with one another through the whole plant ; and we know 
from experiment that these are in their turn in complete connection here and there 
with the cavities of the vessels and with the wood-cells^. The large air-cavities which 
are abundant even in land-plants (in hollow stems, leaves, fruits, &c.), the woody tubes 
(or vessels) and wood-cells, and the usually extremely narrow capillary intercellular spaces 
of the parenchyma, form therefore a system of cavities full of air and in communication 
with one another, which are all closed below at the root, but which open outwardly 
above in the leaves, internodes, &c., through numberless extremely narrow capillary 
openings. 
What was said in paragraph b on the changes which take place in the air contained 
in the cavities of water-plants, applies in general also to that of land-plants ; but the 
equalising of the difference in the pressure at the various parts of a large plant is 
facilitated by the occurrence of vessels, that of the difference between the internal and 
external air by the stomata. This equalisation however proceeds in general extremely 
slowly, because the stomata, in consequence of their small diameter, can allow only small 
volumes of gas to pass through them in a short time. Notwithstanding their unin- 
terrupted connection, there may therefore be considerable differences of pressure and 
great variations in the composition of the internal and external gas, as in water-plants. 
It must also not be forgotten that those layers of tissue in which a rapid interchange of 
gases is proceeding are covered with an epidermis containing a greater number of 
' Large Fungi and Algce have indeed no stomata; but their internal air (among the hyphse) is 
certainly in communication at least in places with the surrounding air by cavities among the super- 
ficial hyphse. The stems of Mosses possess neither internal cavities nor stomata, while their spore- 
capsules possess both. 
^ [The fact to which allusion has been made on p. 682, that namely the air in the vessels of an 
actively transpiring plant is at a lower pressure than tliat of the atmosphere, proves that the cavities 
of the vessels do not communicate with the intercellular spaces. If they did so communicate, such a 
difference of pressure could not possibly arise.] 
