TRANSPIRATION. 1 19 



sion of the gases in the wood-vessels of the plant, and that 

 consequently an increased resistance is offered to the filtra- 

 tion of water into the wood of the root. Further, this negative 

 pressure must also promote the circulation of gases in the 

 plant. Von Hohnel found that, as might be expected, gases 

 readily diffuse from the surrounding tissues into the vessels 

 when the pressure is low, the gases given off by any one part 

 of the plant being thus brought into a closed system of 

 channels which directly communicates with other parts ; and 

 that conversely when there is a considerable root- pressure, 

 the gases in the vessels are forced out of them and pass by 

 diffusion into the neighbouring cells. In the vessels them- 

 selves currents are set up which are due partly to the difference 

 of the composition of the mixture of gases in various parts of 

 the system, and partly to differences of temperature. 



The principal channels, however, in which gases circulate 

 in the tissues of plants are the intercellular spaces. That 

 these form a continuous system throughout the plant com- 

 municating with the external air through the stomata and the 

 lenticels is shewn, not only by microscopical observation, but 

 also by experiments of the kind mentioned in the fifth 

 lecture (p. 71) in which air is forced through leaves, branches, 

 etc. In these, as in the vessels, there are diffusion-currents, 

 and currents due to variations of temperature and probably 

 also to the swaying movements of the leaves and branches. 



In submerged plants, which do not transpire and which 

 have rudimentary fibro-vascular bundles, the intercellular 

 spaces are the only channels in which gases circulate. They 

 are usually very large and are termed air-chambers. It 

 frequently happens that gases (more particularly oxygen) 

 collect in these chambers until the pressure becomes so great 

 that the surrounding tissues are ruptured, and the gases 

 escape in the form of bubbles. 



The mixture of gases contained in the tissues of plants 

 has been analysed in several cases, and, inasmuch as the 

 relative proportions of the gases must vary considerably in 

 correspondence with the varying metabolic processes, the 

 results of the analyses differ widely. The gases found to 

 be present are oxygen, nitrogen, and carbon dioxide. The 



