Lepidium and Linum did not germinate at 0° C., but did germinate at 1.8° C.

Collomia did not germinate at 3° C., but did germinate at 5.3° C. Nigella, Iberis, and Trifolium repens did not germinate at 5.3° C., but did germinate at 5.7° C.

Mays did not germinate at 5.7° C., but did germinate at 9° C. Sesamum did not germinate at 9° C., but did germinate at 13° C. Melon did not germinate at 13° C., but did germinate at 17° C. Malvacea, Gossypium herbaceum; variety not specified: Some cotton seeds on which experiments had been made two years before would not then germinate, but did germinate at this time at 40° C. Raphanus sativus (radish): Lefebure had shown that these seeds germinate at 5° or 6° C. as their minimum temperature.

Triticum (winter wheat), Hordeum (barley), Secale cereale (rye): All of these Gramineæ germinated at 7° C., according to Edwards and Colin, but this is probably not their minimum, for certainly barley will germinate at a lower temperature by prolonging the experiments.

We conclude, therefore, that each species has a minimum temperature at which it germinates, and the ordinary experience of the farmer would suggest this, but in his work one can hardly decide whether seeds sown too early in the springtime are simply retarded by specific low temperatures or whether germination is quite impossible. These present experiments show that if the temperature is too low, then germination is prevented. In calculations on the relation of temperature to vegetation, one must consider only facts deduced from prolonged, constant temperatures. In the study of growth under natural conditions one must consider certain temperatures as useless and ineffective as concerns the germination of certain species of plants. There are, moreover, other facts that show that the same rule holds good for leafing, flowering, and maturing.

According to De Candolle's experiments, the species that require high temperatures as minima for germination are all from warm countries. Such species can not flourish in cold countries, for if they do germinate there this happens too late in the springtime and they can not ripen their fruits before winter. Among the species which germinate at low temperatures there are some that can exist in temperate climates, but these do not extend very far toward polar regions, either for reasons foreign to the germination or else because, having germinated too early, the delicate shoots are killed by frost.

(3) There is for each seed a maximum temperature beyond which germination is impossible. The above experiments determine such maxima approximately as follows:

Nigella does not germinate if the mean temperature exceeds 28° C. Collomia does not germinate if the mean temperature exceeds 28° C.

Trifolium repens: Very few seeds germinate at 28° C., and probably none at 30° C.

Mays: Probably the upper limit is 35° C., although one seed germinated after being exposed to 50° C.

Melon will stand 40° C., but it is probable that above 42° C. germination is impossible.

Sesamum will stand 40° C., and possibly 45° C., the latter being the upper limit.

These upper limits, as I have before said, depend very much on the moisture, and on account of the difficulty of the experiment I have not endeavored to obtain great exactness.

Lepidium and Linum: According to the experiments of Burckhardt, some of these seeds have germinated after an immersion of half an hour in water at 50° C., but not after half an hour in water at 60° C.

Raphanus sativus (radish): Lefebure shows that these seeds germinate in moist earth at a maximum temperature of 38° C.

Triticum (winter wheat), Triticum (spring wheat), Hordeum (barley), Secale cereale (rye), and Avena (oats) germinate perfectly at 40° C., partially at 45° C., and not at all at 50° C.

(4) The range between the maximum and minimum temperatures at which germination is possible differs appreciably for these various species. Evidently a small range is a condition unfavorable to an extensive geographical distribution.

(5) Marked differences are observable between seeds of the same species and coming from the same place. This is well known to the farmer and strongly affected some of the preceding observations. The seeds of the same plant or the same capsule are not identical physically nor chemically. But if the temperature and moisture are those most favorable to germination, many seeds will sprout simultaneously, whereas near the maximum, and especially the minimum, temperature the seeds germinate very irregularly and many of them not at all.

(6) The structure of the seeds, especially the presence and nature of the albumen within them, ought to exert a definite influence, but the small number of species that De Candolle experimented upon does not allow of extensive generalizations.

The species, having little or no albumen-viz, Sinapis, Lepidium, and Linum-germinate at very low temperatures. Those having the next larger amounts of albumen-viz, Nigella, Collomia, and Zea mays-germinate at about 5° C.; but Sesamum, which has but little albumen, requires 10° or 12° C.

At 17° or 18° C. all these seeds germinate well, but the length of time required increases somewhat as the albumen increases, showing that the latter exerts a retarding influence. The order of germina

tion at this temperature is as follows: Lepidium, 1.5 days; Sinapis, 1.7 days; Trifolium, 2.6 days; Sesamum, 3 days; Linum, 3 days; Iberis, 4 days; Zea mays, 3.75 days; Collomia, 5.5 days; Nigella, 6 days; Melon, 9.25 days.

(7) The relation between temperature and the time required for germination is such that the time is shortest at a certain best temperature for each seed and increases to infinity or impossibility as we depart from that temperature toward the maximum and minimum limiting temperatures. All calculations of the sums of daily temperatures, both in geographical botany, in agriculture, and horticulture, are complicated by hypotheses and affected by many causes of inaccuracy, so that De Candolle hesitates to draw very precise conclusions from his laborious experiments. However, he shows that if the duration required for germination as expressed in days is multiplied by the corresponding temperature expressed in degrees Centigrade we shall then obtain much more consistent figures if the temperatures are counted from the minimum for each plant instead of from the zero of the Centigrade thermometer. The tables on pages 32 and 33 give the temperatures and the durations in days, as observed by De Candolle for the species experimented upon by him. For three of these he adopts as the starting point of his calculations the following minimum temperatures-viz, for Lepidium, 1° C.; for Trifolium repens, 5.5° C.; for Sesamum, 11° C.

(8) When seeds are subject to variable temperatures, as occurs to a slight degree in these experiments, and to a still larger degree in nature, the so-called useless or ineffective temperatures may be in fact unfavorable and even retard the germination, since moisture continues to be absorbed into the seed, although the latter can make no use of it.

(9) There is some analogy between the germination of seeds and the hatching of eggs. Thus Millet and Robinet have shown that the hatching of the eggs of the silk worm requires at least a temperature of 9° C., and that as the temperature increases above this the number of days required to hatch diminishes faster than required by a constant sum total, so that at a temperature of 20° C., ten days accomplishes more than twenty days will do at a temperature of 10°. This shows an influence of the minimum temperature similar to that for the seeds of plants.

An entirely analogous case has been worked out by the author with regard to the hatching of the eggs of the grasshopper when deposited in the soil of our western plains. The details of this study will be found in the First and Second Reports of the United States Entomological Commission, and afford an illustration of the possibility of making from meteorological data a prediction as to the hatching

of the eggs of this pest, such as may guide the farmer in his sowing or planting so that the young plant may escape the ravages of the young insects.

INFLUENCE OF TEMPERATURE AND MOISTURE ON GERMINA

TION.

The influence of temperature and moisture on the sprouting of seeds has been studied by Sturtevant at Cornell University (Agr. Exp. Sta., Bull. No. 7), with results generally confirming those of De Candolle. Sprouting occurs better with a uniform than with a variable temperature, so that the method of Quetelet, which requires us to take account of the squares of the temperatures, is no better than that which considers the simple temperature. The rapidity of sprouting diminishes with the decrease of temperature. The percentage of seeds that sprout does not depend upon the uniformity of the temperature. Sprouting takes place more rapidly in a rather dry soil, but a decidedly wet soil is injurious. By soaking the seed before planting it, the interval between planting and sprouting is diminished, but not between soaking and sprouting; hence the total time required, and the total percentage of sprouting seeds is not much affected by the soaking. The exposure to light during germination retards some seeds, but does not affect others. Actual planting in the field may give 50 per cent less germinations than given by similar seeds planted in experimental pots under control.

INFLUENCE OF LIGHT AND HEAT ON GERMINATION.

Pauchon (1880) summarizes the results of the studies of many authors on the relative influence of light and heat on the germination of seeds and the growth of plants. The following section is condensed from him:

Edwards and Colin (1834) state that in their day little was known as to the influence of light and air on the green matter and on the respiration of plants; since then, however, it may be considered as established that the life of a plant varies in proportion to the adaptation of the plant to its surroundings. The study of the influence of light may be said to have begun with Lavoisier, who thought that the light directly combined with certain parts of the plant producing the green leaves and colored flowers, and that without light there could be no life. Similarly Moleschott (1856), at Zurich, affirms that in general everything that breathes or moves draws its life from the light of the sun.

Boussingault (1876), controverting a statement of Pasteur, maintains that the growth of mushrooms and mold in the dark is not an exception but a confirmation of the general rule, and that if the solar

light should be cut off both the plants having chlorophyl, and also the plants that do not have it, would disappear from the surface of the globe.

Berthelot, in his essay on the mechanics of chemistry as based on thermochemistry, shows that the action of the light is demonstrated by the formation of complex chemical effects, isomeric changes, and more complex reactions. For instance, the combination of free oxygen is stimulated in a great many cases by the action of light, as is shown by the bleaching of fabrics of any kind exposed to the air and by the oxidation of volatile oils. All the oxidizing in reactions brought about by the action of light is exothermic-that is to say, there is a loss of energy in the transition from the compound body to its elementary components and a disengagement of heat. The light plays the rôle of a determining agent. On the other hand, when a complex body is built up in the cells of a plant, by drawing in elementary bodies from the atmosphere and soil, the reaction is endothermic, and solar heat is absorbed and rendered latent in the plant.

Sachs, Wiesner, and Mikosh would seem to have established the principle that the formation of the green matter of a plant is not dependent wholly on the light as such, but also demands a certain temperature, varying between 0° and 35° C., for the various plants of Europe. They show also that an increase in the temperature of the atmosphere, with equal increase of light, increases the rapidity of the formation of the chlorophyl up to a certain maximum temperature, and that in proportion as the temperature departs from this favorable maximum, either above or below, the formation of the green matter becomes less and less active, until when the limits 0° or 35° C. are exceeded it ceases altogether. But the temperature most favorable for the formation of chlorophyl under the action of light has but little connection with the temperature that promotes the further action of the chlorophyl after it has been formed within the plant. Thus Timiriazeff (1880) shows that the activity of the chlorophyl consists in the absorption of certain radiations; but in order that these radiations may act it does not suffice merely that they should be absorbed; it is further necessary that there should be a very considerable intensity of heat, in order to furnish to the chlorophyl the definite number of calories necessary for the decomposition of the carbonic-acid gas taken in from the atmosphere.

In general, under ordinary conditions light is indispensable to the formation of chlorophyl. To this general law there are a few apparent exceptions, as follows: The embryos of the genera Pinus and Thuya have their cotyledons colored an intense green at the moment of germination, even when they have been or appear to have been completely deprived of the action of light. So also with a certain number of phanerogams in which the embryo is protected by thick