2 72 THE POPULAR SCIENCE MONTHLY 



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 per- 

 foliata) 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 tempera- 

 ture 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 con- 

 sumption 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 



