May 29, 1885.] 



SCIENCE, 



441 



AQUATIC PLANTS OF SAN DIEGO. 



During the wet spring of 1884 I had an 

 excellent opportunity to note the aquatic flora 

 of this vicinity. Doubtless it seldom reaches 

 such luxuriance ; and in some 3-ears, owing to 

 the scarcity of water, many of the plants cer- 

 tainty make no appearance. 



Surface-water reached an exceedingly low 

 stage in 1883 ; and San Diego was supposed 

 remarkably free from any water-plants, except 

 the wide-spread Azolla, and a few other well- 

 known species. However, the heav}- rains of 

 1884, flooding the entire country, revealed a 

 surprisingly large variety ; and that, too, where 

 one would least expect it, — on the broad, 

 usually dry and barren mesas. 



The surface-geology of large portions of 

 these mesas is characterized by innumerable 

 hillocks, or small mound-like formations, rising 

 from one to four feet above the intervening 

 depressions, and ranging from ten to fifty 

 feet in diameter. The} 7 are generally nearly 

 circular, though often irregular ; and the de- 

 pressions contain in stony places accumula- 

 tions of cobblestones. 



These innumerable hollows naturally become 

 miniature lagoons as soon as heavy rains com- 

 mence ; and soon the leaves of Callitriche are 

 floating upon their surface, while the deeper 

 portions of the little lakes are lined upon the 

 bottom with large patches of Pilularia Ameri- 

 cana, Tillaea angustifolia (Nutt.) , and Elatine ; 

 and along the borders are other minute plants 

 which altogether form a tangled mat of minia- 

 ture luxuriance, exceeding in comparison the 

 vegetation of the largest lakes. Some of the 

 larger pools, longer covered with water, are 

 filled along the edges with junci, sedges and 

 grasses, among which, at the bottom, Isoetes 

 thrives as well as in the northern lakes. 



Later in the season, Downingia pulchella and 

 Pogogyne nudiuscula, with several less con- 

 spicuous species, border the pools ; and still 

 later a new golden Bloomeria, blue Brodiaeas, 

 and other beautiful Liliaceae, are found ; and 

 these, in turn, give way to a few Compositae, 

 preceding the next dry season. 



This year another plant, Marsilia vestita, 

 common to lagoons at high altitudes, and also 

 Ammannia latifolia (L.) and Echinodius ros- 

 tratus (Engelm.), grew abundantly in this 

 vicinity, on the borders of a usually dry flat, 

 near the level of the sea. Other aquatics were 

 found in great quantity throughout the country ; 

 and nearly two dozen species of common water- 

 plants, previously unknown to this section, 

 were added to the local flora. C. R. Orcutt. 



SUNLIGHT AND THE EARTH'S ATMOS- 

 PHERE.^ 



There is, we may remember, a passage in which 

 Plato inquires what would be the thoughts of a man 

 who, having lived from infancy under the roof of a 

 cavern, where the light outside was inferred only by 

 its shadows, was brought for the first time into the 

 full splendors of the sun. We may have enjoyed the 

 metaphor without thinking that it has any physical 

 application to ourselves, who appear to have no roof 

 over our heads, and to see the sun's face daily; while 

 the fact is, that if we do not see that we have a roof 

 over our heads in our atmosphere, and do not think 

 of it as one, it is because it seems so transparent and 

 colorless. 



Now, I wish to ask your attention to-night to con- 

 siderations in some degree novel, which appear to me 

 to show that it is not transparent, as it appears, and 

 that this seeming colorlessness is a sort of delusion 

 of our senses, owing to which we have never in all 

 our lives seen the true color of the sun, which is in 

 reality blue rather than white, as it looks; so that 

 this air all about and above us is acting like a 

 colored glass roof over our heads, or a sort of optical 

 sieve, holding back the excess of blue in the original 

 sunlight, and letting only the white sift down to us. 

 I will first ask you, then, to consider that this seem- 

 ing colorlessness of the air may be a delusion of our 

 senses, due to habit, which has never given us any 

 thing else to compare it with. 



If that cave had been lit by sunshine coming 

 through a reddish glass in its roof, would the per- 

 petual dweller in it ever have had an idea but that 

 the sun was red ? How is he to know that the glass 

 is ' colored,' if he has never in his life any thing to 

 compare it with ? How can he have any idea but 

 that this is the sum of all the sun's radiations (corre- 

 sponding to our idea of white or colorless light) ? Will 

 not the habit of his life confirm him in the idea that 

 the sun is red ? and will he not think that there is no 

 color in the glass, so long as he cannot go outside to 

 see ? Has this any suggestion for us, who have none 

 of us ever been outside our crystal roof to see ? We 

 must all acknowledge in the abstract, that habit is 

 equally strong in us, whether we dwell in a cave or 

 under the sky; that what we have thought from 

 infancy will probably appear the sole possible expla- 

 nation; and that, if we want to break its chain, we 

 should put ourselves, at least in imagination, under 

 conditions where it no longer binds us. 



The Challenger has dredged from the bottom of the 

 ocean fishes which live habitually at great depths, and 

 whose enormous eyes tell of the correspondingly 

 faint light which must have descended to them 

 through the seemingly transparent water. It will 

 not be so futile a speculation as it may at first seem, 

 to put ourselves in imagination in the condition of 

 creatures under the sea, and ask what the sun may 

 appear to be to them; for, if the fish who had never 



1 A lecture delivered at the Royal institution, April 17, 1885, 

 by Prof. S .P. Langlet of Alleghany. From advance sheets of 

 Nature, kindly furnished by the editor. 



