March 2, 1893] 



NATURE 



427 



logical and botanical collections. For two months we had lived 

 at an altitude of over 15,000 feet, soaked by the rains and 

 blinded by the snow and hail, with little or nothing to eat, and 

 nothing to drink but tea, and yet not one of us had a 

 moment's illness from the day we left till we reached our 

 homes again." 



GASES IN LIVING PLANTS.^ 



DLANTS are permeated by the same gases that make up the 

 -*• atmosphere surrounding them : oxygen, carbon dioxide and 

 nitrogen. Nitrogen in the form of a gas is neither used nor 

 generated by any part of plants, unles5 we except the tuber- 

 cles of certain roots, and so it occurs in about the same per- 

 centage inside the plant as outside of it. On the other hand, 

 both oxygen and carbon dioxide enter into combination with, 

 and are liberated from, the plant tissues in varying amounts 

 at different times. The percentage of these two gases in the 

 cavities of the plant vary through a considerable range. In a 

 series of determinations made by Lawes, Gilbert, and Pugb, in 

 England, the oxygen ranged from 3 to 10 per cent., and the 

 carbon dioxide from 14 to 21 per cent, in plants which had 

 been for some time in the dark, while plants which had been 

 standing in sunlight reversed these figures, and gave 24 to 27 

 per cent, of oxygen and 3 to 6 per cent, of carbon dioxide. The 

 two gases, therefore, bear a somewhat reciprocal relation, their 

 sum usually being about 25 to 30 per cent, of the total gas in 

 the plant. 



The variations in the relative amount of oxygen and carbon 

 dioxide are due to two independent processes incident to the 

 life of plants. One of these processes is assimilation, by which 

 all green cells of plants in the presence of sunlight, or its 

 equivalent, such as a strong electric light, absorb carbon dioxiie 

 and liberate oxygen. This process goes on with great rapidity 

 in healthy cells, but is entirely checked upon the withdrawal of 

 light, or when it reaches a certain low intensity. Of course it 

 never takes place in roots, flowers, the central portion of large 

 stems, or other parts which are not green, nor in any fungi or 

 other plants not possessed of green colouring matter. 



The other great cause of disturbance in the relation of oxygen 

 and carbon dioxide in the plant is the process of respiration. 



Respiration in plants is essentially the same as in animals, 

 and consists in a fixation of oxygen and the liberation of carbon 

 dioxide. It fakes place in every living cell, whatever the kind 

 of plant, whatever the part of the plant, and whatever the con- 

 ditions of active existence. The rate of respiration varies with 

 the temperature, the age of the cell, and the nature of the 

 chemical transformations. In normal respiration the amount of 

 oxygen absorbed is approximately the same as the amount of 

 carbon dioxide evolved. There are, however, certain modified 

 forms of respiration in which this does not hold true. 



If living plants be placed in a vacuum, or in an atmosphere 

 deprived of oxygen, it is found that they can still carry on life 

 processes for some time, accompanied with an evolution of car- 

 bon dioxide. The oxygen necessary for this process is obtained 

 from the breaking up of compounds in the cells, and it is there- 

 fore called intramolecular breathing. 



The germination of seeds, which contain a large amount of 

 oil, is somewhat the opposite of this last process. In order to 

 convert the fat into a more directly serviceable food material for 

 the plant, a large amount of oxygen enters into the new com- 

 bination, for which there is no equivalent amount of gas liberated. 

 It consequently comes about that oily seeds in germinating ab- 

 sorb a far larger amount of oxygen than they liberate of carbon 

 dioxide. This is known as vincular breathing. 



Another variation from normal respiration is known as insolar 

 breathing, and which, with still some other modifications, I need 

 not stop to explain. To this brief statement of plant respira- 

 tion must be added that much yet remains to be discovered re- 

 garding the details of the processes. 



Assimilation and respiration are the two great causes which 

 disturb the relative volume of the two variable gases in plants. 



We shall now turn to the movement of the same two gases, 

 oxygen and carbon dioxide. There has never been a disposition 

 as in the case of many other plant phenomena, to explain the 

 movement of gases upon any other than purely physical prin- 

 ciples. We have therefore to do simply with the question of 



Reprinted from the American Naturalist for February. 



NO. I 2 18, VOL. 47] 



the aids and hindrances to the establishment of an equilibrium 

 between the gases inside and outside the plant, irrespective of 

 whether the cells are alive or dead. 



It has already been stated that the relative amounts of oxygen 

 and carbon dioxide inside the plant are usually very different, 

 and that within a few hours the relation of the two may be com- 

 pletely reversed. To this may be added that the pressure of the 

 gases inside the plant is sometimes more, sometimes less than 

 that of the atmosphere outside the plant, almost never the same. 

 Hales observed in his early work that a mercury gauge con- 

 nected with the inside of the trunk of a tree showed an mternal 

 pressure when the hot rays of the sun warmed the trunk. This 

 was largely due, undoubtedly, to an expansion of the gases in 

 the trunk, by the heat. Such an excess of pressure in water 

 plants is very common, although due to other causes. It may 

 readily be shown by breaking stems under water, when bubbles 

 of gas will be liberated, as undoubtedly many have noticed in 

 gathering water lilies, or other water plants. 



On the other hand, the pressure of the gas inside the plant 

 may be less than on the outside. This has long been recognised, 

 but was best demonstrated by Von Hohnel in 1879, to whom it 

 occurred to cut off stems under mercury. In doing so the mer- 

 cury rose to a considerable height in the vessels of the stem, and 

 as mercury is without capillarity, this can only be ascribed to 

 the greater pressure of the outside air, or in other words, to a 

 partial vacuum in the plant. 



An observation was made by Hales, which we may use to 

 illustrate how such a negative pressure, as it has been called, 

 can be brought about. He cut off a branch, fastened an empty 

 tube to the cut end, and plunged the other end of the tube into 

 a liquid. He found that as evaporation of moisture from the 

 leaves took place, the liquid was drawn up into the empty tube. 

 This phenomenon can now be explained more satisfactorily than 

 could be done at that early day. By evaporation the liquid 

 water inside the plant escapes in the form of vapour, and the 

 space it occupied is filled by the gases, thus rarifying them. 

 This rarifaction may go on in uninjured plants until the internal 

 pressure is greatly reduced. But in the experiment, the pres- 

 sure is equalised by the rise of the liquid in the tube. A later 

 modification of Hales' experiment is to use a forked branch, 

 place the cut end in water to give a continuous supply of mois- 

 ture for transpiration, and attach the empty tube to one of the 

 side forks of the stem, cut away for that purpose. 



It is self-evident that such condensation and rarifaction of 

 the gases in the plant could not take place if the cell walls were 

 readily permeable to gases. Thus it comes about that one of 

 the most important topics in connection with the movement of 

 gases in the plant, is the permeability of tissue walls of various 

 kinds, and especially those constituting the surface covering of 

 plants. 



I shall not attempt to conduct you through the tangle of sup- 

 position and fact, errors in experiments, correct and incorrect 

 conclusions, and the general confusion which has come from the 

 labours of physicists, chemists and botanists for the last twenty- 

 five years, during which the subject has received particular 

 attention. The results of the later work have been to cast 

 grave doubts upon the correctness, or at least the interpretation 

 of some of the experiments most relied upon heretofore. Never- 

 theless many points still lie open for verification, and untouched 

 parts of the subject await investigation. 



In the earlier days it was found that the leaves and young 

 stems of plants have their epidermis more or less well supplied 

 with minute openings, called stomata, or breathing pores, 

 which communicate with small air cavities in-ide, which in turn 

 branch out among the cells into a network of minute passages 

 rarifying throughout the plant. This intricate network of in- 

 tercellular passages affords an air communication throughout 

 the *hole plant, and connects directly with the outside atmos- 

 phere through the stomata. Subsequent to the discovery of 

 stomata, it was ascertained, that in stems more than one year 

 old, the stomata are replaced by another kind of opening, known 

 as lenticels, which in some form are doubtless to be found in 

 the bark of shrubs and trees of whatever age. 



Gases stream into and out of the plant through the stomata 

 and simpler lenticels, according to the law governing the 

 movement of gases through minute openings in thin plates. The 

 rate of movement is accordingly proportional to the square roots 

 of the density of the mixing gases. Such a movement of gases 

 is known as effusion. 

 The movement by which gases pass from one part of the 



