of the light and the action of the chlorophyl of the leaves. As we pass from the pole to the equator the luminous intensity of the sunlight increases from a hundred to a thousand, but its duration diminishes during the growing season from a hundred at the poles to fifty at the equator. Among the special investigations into the action of sunlight we note that of Timiriazeff (1877), who has shown that a very intense light, after traversing a certain thickness of green-leaf cells, has no further action on the phenomena of the reduction or decomposition of carbonic-acid gas; in other words, it acts the same as darkness would do. On the other hand, Paul Bert, by exposing plants to the action of light which had been sifted through a solution of 'chlorophyl, invariably found that the development of the green matter of the leaf was completely arrested; inversely, he found the green matter produced to its normal amount when the plant received only light that had been filtered through a solution of iodine in bisulphide of carbon, which solution, as we know, cuts off all visible rays, but allows the red and infra-red to pass through with great freedom. This would seem to demonstrate that chlorophyl is formed by the action of the red portion of the spectrum.

As to the effect of light on the germination of seeds, Pauchon (1880) gives a critical summary of views by different authors, from which we condense the following:

Miesse (1775), from observation on the Camelina (Myagrum sativum), concludes that the seeds grow in darkness the same as in full daylight, and that light does not seem to influence this stage of vegetation.

Sénébier (1782), from observations on seeds of lettuce and beans, some of which were, exposed to the full sunlight, others to sunlight after filtering through a thickness of water, others in the dark, and others in red, violet, and yellow light, respectively, reached the conclusion that light was injurious; but his results were not decisive, because of his neglect to observe exactly the temperatures under different conditions.

Ingenhousz (1787) exposed an equal number of mustard seeds in places receiving different amounts of light. He himself concluded that the light of the sun is as injurious to vegetation at the beginning of its life as it is advantageous to vegetation in the fullness of its life. But a more careful consideration of Ingenhousz's experiments shows that the moisture and the temperature in his several localities varied so much as to prevent any serious conclusion as to the action of light itself.

Bertholon (1789), in an article on the effect of electricity, shows

that up to that time it had not been proven whether the germination of the seeds was affected by light or by humidity. His own experiments convinced him that the latter was more important.

Sénébier (1800) made additional experiments on peas and beans, sowing them in sponges, which were kept equally moist, all inclosed under glass covers, so that no evaporation could take place. Some were exposed to sunlight and some were kept in the dark, but those which were in the dark germinated much sooner than those in the light. But in such experiments as these the sources of error are numerous, and the fact that there was no renewal of the air under these covers was especially unfavorable to germination. In fact, Leclerc (1875) has shown that under the influence of mercurial vapor, as it existed in Sénébier's experiments, a large portion of seeds are killed, so that with our present knowledge we can not accept Sénébier's conclusions.

Lefébure (1800), having finally accepted the conclusions of Sénébier and Ingenhousz relative to the injurious influence of light on germination, repeated the experiments, but also observed the temperatures more carefully, and in addition sought to determine the effect of light that had passed through plates of white, green, black, red, and blue glass; but he added little to our knowledge, although he himself concluded that the seeds under white glass were retarded. Th. de Saussure (1804) endeavored to ascertain whether the influence observed by others was due to light or heat, and he concluded that nothing demonstrates that light has an injurious influence independent of the heat that accompanies it.

Keith (1816) made no observations himself, but controverted the conclusions of De Saussure.

Boitard (1829) sowed the auricula seeds in three flower pots, but the conditions as to temperature and moisture are not sufficiently known to justify us in drawing any conclusion.

A. P. de Candolle (1832) says:

I do not deny that darkness may be useful in germination, but I do deny that it is necessary to think that light has no action on germination. Analogy indicates this, theory confirms it, and experience demonstrates it.

According to De Candolle, light favors the decomposition of carbonic acid, but germination demands the formation of carbonic acid; therefore darkness will favor germination. This theory thus enunciated by De Candolle has been accepted by many authors without proper experimental basis.

Ch. Morren (1832) experimented upon water cresses grown under different colored glasses. He concluded that as darkness favored germination, so the individual colors of the spectrum, acting each by itself, have a special influence that favors germination in such a way that

those colors that have the greatest illuminating power are those that least favor germination.

Ad. Brongniart (1832) announced as the results of his experiments that the retarding influence of light depends not only on the illuminating power of the colored light, but on the relative quantity of white light that passes through the different colored glasses. In all these experiments the seeds were several millimeters below the surface of the soil, so that the colored lights did not affect the seeds directly, but indirectly through the soil whose temperature and moisture and evaporation may easily be of predominating importance.

Ph. A. Pieper (1834), Meyen (1837), Zantedeschi (1846), and Belhomme (1854) have all experimented on the growth of seeds under colored glasses; but the sources of error incident to this method of observation prevent us from drawing any conclusion as to the influence of light itself.

Ville (1865) says that the injurious effect of solar radiation on germination is the result of the heat only and that the effect of the light is inappreciable. For aquatic plants whose seeds germinate in the water, darkness seems decidedly favorable to germination, but it acts only in an indirect manner by preventing the warming of the water and the disengagement of the oxygen that is dissolved in this

water.

Charles Darwin (1877) says that certain species of seeds do not grow well when they are exposed to the light, even the diffuse light of

a room.

Duchartre (1877) considers the action of darkness as a secondary influence, useful but not at all essential and concerning which there has been too much exaggeration.

Faivre (1879) has shown that the appearance of the primordial latex occurs at a moment when the radicle is only a few millimeters long and when the cotyledons are still inclosed in the seed envelopes and have not yet received the action of light. He notes that under a yellow light obtained by transmitting sunlight through a solution of bichromate of potash the seeds develop their chlorophyl and their latex more rapidly, and consequently have a shorter period of germination than under a blue light obtained by transmitting sunlight through a solution of the ammoniacal oxide of copper.

Detmer (1880) has consecrated an extensive work to the study of the germination of seeds, and states that concerning the action of light we are still ignorant as to whether it is direct-that is to say, whether it stimulates the storing up of new substances in the vegetable tissue or whether, on the contrary, it strengthens the persistence within the cells of some special process having a more or less intimate relation to the phenomena of growth and which can only

proceed in darkness. Detmer adds a few historical references, viz, Humboldt (1794), according to whom seeds sprout more easily in darkness than in light; Fleischer (1851), Heiden (1859), and Nobbe (who all consider solar rays as having no action on the seeds), and, finally, Hunt (1851), who considers that light retards germination.

After this preliminary historical survey, Pauchon communicates the results of his own experiments as to the influence of light on germination on the following twenty-two species of plants:

After deducting doubtful results or failures Pauchon gives the following conclusions (see p. 131 of his work above quoted):

(1) In 22 experiments germination occurred first in the light; in 26 experiments it occurred first in the dark.

(2) Five times we obtained duplicate results favorable to the light for the same species of plants (Arachis, Zea mays, Dolichos, Sinapis, and Linum). Eight times these duplicate results were favorable to specimens kept in the dark (Helianthus, Delphinium, Pancratium, Ricinus, and Papaver). In one case (Linum) two results were obtained favoring light and two favoring darkness.

(3) Among the 22 species of plants used in the experiments 14 gave mixed results equally favorable whether placed in the light or the dark.

(4) Among the 8 other varieties only 1 gave negative results (Coffea); 3 gave results favorable to light (Cucurbita, Spilanthes, and Carthamus); 4 gave results favorable to darkness (Delphinium, Pancratium, Lepidium, and Nigella).

It appeared to Pauchon impossible to draw any conclusion whatever from these facts. Should we be astonished at this? The problem is certainly much more complex than appears at first sight.

There is every reason to suppose, for example, that the action of light is not the same under all the conditions of temperature which obtained during these experiments. Here again, however, we are confronted by the unknown; because, in order to draw from these researches the consequences which might flow from them it would be necessary to know precisely the thermic conditions favorable to the germination of each species. Unfortunately this is a very important gap to be still filled up, as the work accomplished in this direction gives only approximate results limited to a very small number of different kinds of seeds. On the other hand, looking to facts of another order, mentioned further on in this work, we think that we may be allowed to suppose that the influence of light can only be favorable to germination when it acts at temperatures below that which is most favorable to germination. A considerable number of observations already cited would seem to be in accord with this view of the subject. But unfortunately the many contradictions that we observed in our results do not allow us to accept this opinion as based upon a solid foundation.

Pauchon then goes on as follows:

Another reason, however, induces me to admit, only with many reserves, the results of experiments whose critical epoch is the visible development of the embryo. A method based on this special observation does not appear to me capable of furnishing a really scientific basis for the determination of the question before us. The process of germination is not, in reality, as simple a phenomenon as the greater number of botanists, perhaps too easily, take for granted. Its complexity is even so great that one can not judge of the actual development of the germ of the plant and of the degree of its physiological activity by the external characters observable by the eye, such as the bursting of the spermoderm and the more or less rapid protrusion of the radicle. I do not hesitate to say, according to observations frequently repeated, that this is an empirical process and entirely deceptive in the particular case that we are dealing with. Although it may be capable of furnishing valuable results when we wish to judge of the influence of some one of the fundamental conditions of germination, it becomes utterly insufficient when it is a question of observing the more delicate and fugitive influences, such as that of light. I have, in fact, in the course of chemical researches, given in the next chapter, demonstrated that for the same stage of apparent development the absorption of oxygen by the seeds in the process of germination varies to a large extent with the temperature, and has no relation to the external growth of the embryo. It is, however, not surprising that the development of the embryo continues in the interior of the seed for a much longer time in one seed than in another of identical appearance; the unknown and variable relation between the reserved nutrition and the rudimentary vegetable is probably the explanation of these hitherto unexplained peculiarities. Although the researches given in this chapter do not give any positive result on the subject of my work. I have preserved them and pub