Page:Popular Science Monthly Volume 74.djvu/589

Rh species in different regions is very clearly related to the amount of sunlight. Northern Europe, New England and the northeastern part of the Mississippi Valley have less sunshine than the great western plateau, the Rocky Mountain region, Italy and California. There are some species of cleistogamous plants on the plateau and in the Rocky Mountain region; there are more on the two slopes of the Alleghanies and in Europe. Only a species or two have been found in central California, and these live in the dim light of virgin redwood forest.

Animal physiologists have not yet shown, I believe, that the breeding-seasons of animals generally mark the reaction of these animals to external influences. There is obviously every reason why birds should not mate till the rigors of a severe winter are over. The breeding-season of frogs and toads must coincide with the season of abundant water in ponds and pools. But I am inclined to believe that where there are no seasonal differences, or only very slight ones, in light, warmth, rainfall, there are only slight differences in the habits of plants and animals. Sea-urchins, for example, like many of the sea-weeds, have no regular breeding-seasons; the changes are slight in the water which nearly always covers them. On the other hand, land animals, subject to the more pronounced changes in their habitat, have their cycles of vital processes to correspond.

Light stimulates flowers to form; it stimulates the violet to develop one kind or another according to the amount of light. Light influences the growth of leaves and stems by its direction quite as much as by its amount. The direction from which light comes determines also where and how a plant part shall form. Vertical leaves, like those of onion and eucalyptus, are alike both structurally and superficially on the two sides. Horizontal or oblique leaves evidently differ on their two faces. Light has much to do with this difference. The reproductive stage of the fern is a small, flat, leaf-like plant, usually growing closely applied to the soil, its upper side lighted, its under side dark. If, for purposes of experiment, the light is made to come from below or from one side, instead of from above, the reproductive organs form, as before, on the side away from the light. They always form on the dark side, whether this is above or below, or more or less vertical. Light and not gravity is here the formative influence, stimulating the reproductive organs to develop and determining by its direction the side of the plant which shall bear them. Sometimes there may be a conflict of influences. If there is no dark side, because the plant is equally illuminated on both sides, the reproductive organs will form equally on the two sides.

If the direction of illumination determines where the reproductive organs of these small fern-plants form, may it not also influence the shape of the plants themselves? It may. There are many small leaf-like plants, allied to the mosses and ferns, growing against soil, or