MOVEMENTS OF GASES IN PLANTS. 693 



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 difl^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 

 diff"usion 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 diffusion 

 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 w^hich 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 Loth. 



^ [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 that 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.] 



