« PreviousContinue »
needs in this respect are even more urgent now than they were thirty years ago, and I can not do better than to reprint and indorse the following appeal first made in an address by A. de Candolle in 1866 :
It appears to me, however, that botanic gardens can be made still more useful in carrying out physiological researches. For instance, there is much yet to be learned on the mode of action of heat, light, and electricity upon vegetation. I pointed out many of these deficiencies in 1855 in my Géographie Botanique Raisonnée. Ten years later Prof. Julius Sachs, in his recently published and valuable work on Physiological Botany, remarks much the same deficiencies, notwithstanding that some progress has been made in these matters. The evil consists in this, that when it is desired to observe the action of temperature, either fixed or varied, mean or extreme, or the effect of light, it is exceedingly difficult, and sometimes impossible (when observations are made in the usual manner), to eliminate the effects of the constant variations of heat and light. In the laboratory it is possible to operate under more exactly defined conditions, but they are rarely sufficiently persistent; and the observer is led into error by growing plants in too contracted a space, either in tubes or bell glasses. This last objection is apparent when it is wished to ascertain the influence of the gases diffused in the atmosphere around plants, or that of the plants themselves upon the atmosphere.
Place plants under a receiver, and they are no longer in a natural condition; leave them in the open air, and the winds and currents, produced at each moment of the day by the temperature, disperse the gaseous bodies in the atmosphere. Everyone is aware of the numerous discussions concerning the more or less pernicious influence of the gases given off by from certain manufactories. The ruin now of a manufacturer, now of a horticulturist, may result from the declaration of an expert; hence, it is incumbent on scientific men not to pronounce on these delicate questions without substantial proof.
With a view to these researches, of which I merely point out the general nature, but which are immensely varied in details, I lately put this question : “ Could not experimental greenhouses be built, in which the temperature might be regulated for a prolonged time, and be either fixed, constant, or variable, according to the wish of the observer ?” My question passed unnoticed in a voluminous work where, in truth, it was but an accessory. I renew it now in the presence of an assembly admirably qualified to solve it. I should like, were it possible, to have a greenhouse placed in some large horticultural establishment or botanic garden, under the direction of some ingenious and accurate physiologist and adapted to experiments on vegetable physiology; and this is, within a little, my idea of such a construction :
The building should be sheltered from all external variations of temperature, to effect which I imagine it should be in a great measure below the level of the ground. I would have it built of thick brickwork, in the form of a vault. The upper convexity, which would rise above the ground, should have two openings—one exposed to the south, the other to the north-in order to receive the direct rays of the sun, or diffused light. These apertures should each be closed by two very transparent glass windows, hermetically fixed. Besides which, there should be on the outside means of excluding the light, in order to obtain complete darkness, and to diminish the influence of the variations of temperature when light is not required. By sinking it in the ground, by the thickness of its walls, and by the covering of its exterior surfaces with straw, mats, etc., the same fixed degree of temperature could be obtained as in a cellar. The vaulted building should have an underground communication with a chamber containing the heating and the electrical apparatus. The entrance into the experimental hothouse should be through a passage closed by a series of successive doors. The temperature should be regulated by metallic conductors, heated or cooled at a distance. Engineers have already devised means by which the temperature of a room, acting on a valve, regulates the entry or exit of a certain amount of air, so that the heat regulates itself. Use could be made of such an apparatus when necessary.
Obviously, with a hothouse thus constructed, the growth of plants could be followed from their germination to the ripening of their seeds, under the influence of a temperature and an amount of light perfectly definite in intensity. It could then be ascertained how heat acts during the successive phases from sowing to germination, from germination to flowering, and from this on to the ripening of the seed. For different species various curves could be constructed to express the action of heat on each function, and of which there are already some in illustration of the most simple phenomena, such as germination, the growth of stems, and the course of the sap in the interior of certain cells. We should then be able to fix a great number of those minima and maxima of temperature which limit physiological phenomena. Indeed, a question more complicated might be investigated, toward the solution of which science has already made some advances, namely, that of the action of variable temperatures; and it might be determined if, as appears to be the case, these temperatures are sometimes beneficial, at other times injurious, according to the species, the function investigated, and the range of temperature. The action of light on vegetation has given rise to the most ingenious experiments. Unfortunately these experiments have sometimes ended in contradictory and uncertain results. The best ascertained facts are the importance of sunlight for green coloring, the decomposition of carbonic-acid gas by the foliage, and certain phenomena relating to the direction or position of stems and leaves. There remains much yet to learn upon the effect of diffused light, the combination of time and light, and the relative importance of light and heat. Does a prolonged light of several days or weeks, such as occurs in the polar regions, produce in exhalation of oxygen, and in the fixing of green matter, as much effect as the light distributed during twelve-hour periods, as at the equator? No one knows. In this case, as for temperature, curves should be constructed, showing the increasing or diminishing action of light on the performance of each function; and as the electric light resembles that of the sun, we could in our experimental hothouse submit vegetation to a continued light.
A building such as I propose would allow of light being passed through colored glasses or colored solutions, and so prove the effect of the different visible or invisible rays which enter into the composition of sunlight. For the sake of exactness nothing is superior to the decomposition of the luminous rays by a prism, and the fixing the rays by means of the heliostat. Nevertheless, a judicious selection of coloring matters and a logical method of performing our experiments will lead to good results. I will give as proof that the recent most careful experiments concerning the action of various rays upon the production of oxygen by leaves and upon the production of the green coloring matter have only confirmed the discoveries made in 1836, without either prism or heliostat, by Professor Daubeny, from which it appears that the most luminous rays have the most power, next to them the hottest rays, and lastly those called chemical.
Doctor Gardner in 1813, Mr. Draper immediately after, and Dr. C. M. Guillemin in 1857, corroborated by means of the prism and the heliostat the discovery of Doctor Daubeny, which negatived the opinions prevalent since the time of Senebier and Tessier, and which were the results of erroneous experiments. It was difficult to believe that the most refrangible rays, violet, for instance, which act the most on metallic bodies, as in photometrical operations, should be precisely those which have least effect in decomposing the carbonicacid gas in plants and have the least effect over the green matter in leaves. Notwithstanding the confirmation of all the experiments made by Doctor Daubeny, when repeated by numerous physicists and by more accurate methods, the old opinions, appearing more probable, still influenced many minds till Prof. Julius Sachs, in a series of very important experiments, again affirmed the truth. It is really the yellow and orange rays that have the most power, and the blue and violet rays the least, in the phenomena of vegetable chemistry, contrary to that which occurs in mineral chemistry, at least in the case of chlorid of silver. The least refrangible rays, such as orange and yellow, have also the twofold and contrary property, such as pertains also to white light, and which produces the green coloring matter of leaves or bleaches them according to its intensity. It is these, also, which change the coloring matter of flowers when it has been dissolved in water or alcohol. Those rays called chemical, such as violet and the invisible rays beyond violet, according to recent experiments confirmatory of those of ancient authors—those of Sebastian Poggioli in 1817 and those of C. M. Guillemin-have but one single well-ascertained effect, that of favoring the bending of the stem toward the quarter from which they come more decidedly than do other rays; yet that is an effect perhaps more negative than positive if the flexure proceeds, as many still believe, from what is going on on the side least exposed to the light.
The effect upon vegetation of the nonvisible calorific rays at the other extremity of the spectrum has been but little studied. According to the experiments we have on this subject, they would appear to have but little power over any of the functions; but it would be worth while to investigate further the calorific regions of the spectrum by employing Doctor Tyndall's process—that is, by means of iodine dissolved in bisulphide of carbon—which permits no trace of visible light to pass.
How interesting it would be to make all these laboratory experiments on a large scale! Instead of looking into small cases or into a small apparatus held in the hand and in which the plants can not well be seen, the observer would himself be inside the apparatus and could arrange the plants as desired. He might observe several species at the same time-plants of all habits, climbing plants, sensitive plants, those with colored foliage, as well as ordinary plants. The experiment might be prolonged as long as desirable, and probably unlooked-for results would occur as to the form or color of the organs, particularly of the leaves.
Permit me to recall on this subject an experiment made in 1853 by Professor von Martins. It will interest horticulturists, now that plants with colored foliage become more and more fashionable. Professor von Martins placed some plants of Amaranthus tricolor for two months under glasses of various colors. Under the yellow glass the varied tints of the leaves were all preserved. The red glass rather impeded the development of the leaves and produced at the base of the limb yellow instead of green; in the middle of the upper surface, yellow instead of reddish brown, and below, a red spot instead instead of purplish red. With the blue glasses, which allowed some green and yellow to pass, that which was red or yellow in the leaf had spread, so that there only remained a green border or edge. Under the nearly pure violet glasses the foliage became almost uniformly green. Thus, by means of colored glasses, provided they are not yellow, horticulturists may hope to obtain at least temporary effects as to the coloring of variegated foliage.
The action of electricity on foliage is so doubtful, so difficult to experiment upon, that I dare hardly mention it; but it can easily be understood how a building constructed as proposed might facilitate experiments on this subject. Respecting the action of plants on the surrounding air and the influence of a certain composition of the atmosphere upon vegetation, there would be by these means a large field open for experiments. Nothing would be easier than to create in the experimental hothouse an atmosphere charged with noxious gas and to ascertain the exact degree of its action by day and by night. An atmosphere of carbonic-acid gas might also be created, such as is supposed to have existed in the coal period. Then it would be seen to what extent our present vegetation would take an excess of carbon from the air, and if its general existence was inconvenienced by it. Then it might be ascertained what tribes of plants could bear this condition and what other families could not have existed, supposing that the air had formerly had a very strong proportion of carbonic-acid gas.
In hopes of realizing this idea of a complete botanic laboratory, the author spent his vacation of 1893 in the botanic gardens and greenhouses of Harvard University. On his return to Washington Professor Riley kindly offered him every convenience and space in the insectary of the Department of Agriculture. His 300 experimental plants of wheat and maize were, therefore, brought hither from Cambridge, Mass. But unforeseen difficulties arose, and it is to be hoped that the idea of an experimental laboratory for botanic study may be carried out by abler hands.
INFLUENCE OF UNIFORM TEMPERATURE ON GERMINATION OF
The results of his own experiments on the germination of seeds at different temperatures were published by De Candolle (1865). His object was to determine the effect of long exposures at low temperatures as compared with short exposures at high temperatures. He eliminated various sources of complication and extended the observations made by Burckhardt (1858). Great pains were taken to keep the seeds at a uniform temperature; the water with which they were wetted was previously brought to the temperature required by the experiment. The first wetting was quite copious. The seeds were first covered with a thin layer of sand and the wettings frequently washed them bare, but no difference was observable in the epoch of germination for naked and covered seeds, showing that the temperatures in the inclosures were very uniform. The thermometers were carefully reduced to a standard Centigrade and their readings are probably correct within a tenth of a degree. The moment of germination is a delicate point to fix and is somewhat arbitrary. The embryo changes within the seed before any change shows itself on the outside. De Candolle takes as the moment of germination that when, the spermoderm being broken, the radicle begins to issue forth. Burckhardt in his experiments took as the epoch of germination the moment when the cotyledons show themselves; but in De Candolle's opinion this is rather an epoch of vegetation than the epoch of germination. It would perhaps be well to consider this phenomenon when we compare the same species under different conditions; but it varies very much from one species to another, since certain plants remain for a long time recurved under the earth or with their cotyledons imprisoned in the remnants of the spermoderm. The seeds experimented on were as follows: Cruciferæ .......... Lepidium sativum.
Do...--------- Sinapis alba.
Do..... Iberis amara.
Zea mays, var. precoce.