Hence we might draw the general conclusion, that all southern plants which are unable to endure our winter have large cells, and that, at the North, only such plants can be naturalized as answer to the requirement of small cells. As in Nature there are no aims, but only necessity, we may also hence conclude that a low temperature is favorable to the development of small cells. We have here, furthermore, an explanation of the hairy coats of animals. Animals which live in the North have all a thick coat, while those living in the south have a thin one. The mammoth was covered with hair 12 inches long, while his descendant, the elephant, who lives only in southern climates, is almost naked. Animals coming from the south, and acclimatized in the north, acquire hair, and vice versa. At the poles the fox wears his winter-coat the whole year through. In Sweden his coat remains for 10 months; in Germany, for 6 months; farther south, 3 months—until at last it is entirely dropped. No one will here discover an aim, but rather this necessary consequence, that a lower temperature produces a growth of hair in some way unknown to us. The same is true as to the development of cells. If, as a general rule, a warmer temperature necessitates larger cells, then the plants of southern regions will perish from the frost of northerly latitudes. The leaf of the potato-vine can never endure frost; but it is only in early spring that the plant can be visited by frost in temperate climates, and there is no frost in summer, while in autumn the tubers are protected by the soil. The young branches of the oak and beech (two trees belonging, indeed, to our climate) are quite as little able to endure the frost, and suffer from it severely during the night in spring. On the other hand, the spicules of the pine and the sword-shaped leaves of the yucca stand the severest cold of our winters.
As regards the temperature of those portions of plants (sprouts of vines, potatoes, etc.) which are killed by the spring frosts, we have no definite knowledge. It is probable that these parts become, by radiation, considerably colder than the shining bulb of the thermometer, and that they do not share in the temperature of the air, but fall to a lower temperature by radiation. In cloudy nights, when the thermometer shows 30° or 31° Fahr., nothing freezes, though the contrary takes place on bright nights. But here, too, the smallness of the cells appears to lower the freezing-point of water some few degrees.
Yet, in thus bringing into very probable relation two different facts, viz., the non-congelation of pupæ? and leaves, and the fluidity of molten sulphur and of mist-particles, we have no complete explanation of the phenomenon. Such an explanation would show why it is that small particles have a different freezing-point from large ones of the same substance. This would require a very profound acquaintance with the nature of the molecular motion of heat, as also of chemical affinity.—Gaea.
