6l6 MOLECULAR FORCES IN THE PLANT. 



but undergo change within them, and when once collected in the spaces, they are still 

 further influenced by 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 diffuse 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 

 different 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 

 dark plant. 



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 their inner cavities, when present ^, com- 

 municating 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 equal- 

 ising of the difference in the pressure at the various parts of a large plant is facilitated by 

 the occurrence of ducts, 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 uninterrupted connec- 

 tion, 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 gas is proceeding 

 are covered with an epidermis containing a greater number of stomata than those 

 which require a less active interchange in consequence of slower growth and assimi- 

 lation. In addition to this, organs with a thin cuticle are better adapted to bring about 



^ Large Fungi and Algre have indeed no stomata ; but their internal air (among the hyphae) is 

 certainly in communication at least in places with the surrounding air by cavities among the super- 

 ficial hyphte. The stems of Mosses possess neither internal cavities nor stomata, while their spore- 

 capsules possess both. 



