Page:Popular Science Monthly Volume 86.djvu/276

272 greater contrast in appearance, size, texture and behavior, than is offered by the two commonest and most characteristic weeds of the two seasons, the two climates, of this region, namely miner's lettuce (Montia perfoliata) and tar-weed (Hemizonia luzulaefolia).

Miner's lettuce, so named because used in the early gold-mining days of California as a salad, grows in the rainy season, when the temperature is low, often below freezing at night, the humidity high, and the soil wet and soft. Its tender, fleshy, but not thick leaves forming a cup upon a succulent stem which is carried on small and shallow roots, are traversed by slender and simple vascular bundles, and the supporting tissues are slight and weak. Its growth is directly proportional to the available and retainable moisture, for it can hold little water against dry air. In a season of scanty rainfall miner's lettuce is short and small, presenting almost a wizened appearance, and as the dry season comes on it droops, dries and disappears.

Tar-weed, so-called because of the odor of the secretion from the glandular hairs borne on its small dry leaves and the slender woody stem and branches, is a well-rooted summer weed, occupying the grain fields after the crop is harvested or continuing long after the native grasses are dry and dead in the caked soil, growing and blooming till the rains come to soften it and to start its successors. It reaches its best development in dry and solid soil, dry air and daily sunshine. Its consumption of water is probably not less than that of miner's lettuce, but its roots can get water and the rest of its body can hold it, in soil and air so dry that miner's lettuce would shrivel and die. Or, to express a more general truth, water determines the character of the vegetation of the succeeding seasons.

Between the plants of the desert and those growing in the spray of a waterfall one may find all gradations, not only within the limits of the state, but often within the limits of an afternoon's walk. Can one do the like elsewhere on this continent or in Europe?

From a study of these conditions there should come clarity to our conceptions of the relations of water and plants, and ultimately such an extension of our knowledge of these relations as will lead not only to clarity, but to completeness.

Water, as a clear and liquid mass, or very finely divided and greatly diluted by the air, we regard as nearly perfectly transparent, though we know that even the clearest water permits the penetration of light for only comparatively short distances beneath the surface. Cloud and fog, less finely divided water than that which we record as the humidity of the air, are far from translucent. We are beginning, as a result of studies of light in very dry air, to suspect that we have underestimated the influence of water upon the quality and the amount of light available for plants in food-manufacture and acting upon them as a stimulus to