Page:Popular Science Monthly Volume 84.djvu/421



HE simpler forms of plants in earlier geologic times lived in swamps and along seashores and under the equable conditions furnished did not attain anything beyond a primitive and elementary development. The chief bar to escape from the restricted moist habitats consisted in the fact that the life cycle of the plant included alternating generations in one of which, the sexual generation or gametophyte, reproduction was possible only in the presence of water. Finally, however, the spore which gave rise to this sexual generation began to germinate in place on the other generation and the resulting gametophyte was produced, and remained enclosed in the tissues of the sporophyte as it is among the seed-plants of the present day.

The domination of the sporophyte in this manner vastly increased the possibilities of evolutionary development, and when this plastic self-contained type of plant began to move out over the broad spaces of the world, all the ranges of temperature afforded by the earth's surface, as well as of moisture, illumination, concentration of the solutions in the soils, alkalinity, etc., were encountered to which to-day the manifold types of plants stand in a delicate adjustment.

Water was the chief determining factor when the vegetal organism was in a separated-generation stage, and it continued to be the most potent agency in evolution and differentiation as the new combined individual moved away from the swamps and shores to the occupation of the drier slopes of valleys and mountains and finally into the most arid of deserts.