FORMATIVE INFLUENCES 585 



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 Eocky Mountain region, Italy and California. There are 

 some species of cleistogamous plants on the plateau and in the Eocky 

 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 breed- 

 ing-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 v/ater 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 de- 

 velop 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 de- 

 termines 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 repro- 

 ductive 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 de- 

 termining 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 repro- 

 ductive 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 



