250 



The Plant World. 



to saturation; a range in water from the point of saturation 

 through salt, brackish, and fresh, to the rain and fog which 

 bathe the leaves and the lower plants; a range in light from the 

 gloom of heavy clouds to the day-long sunshine, the brilliancy 

 of which can not be expressed in definite terms. 



Turning now to the plants living under these conditions, 

 let me begin at and near sea-level. So far as the proportion of 

 salt is concerned, the water of this part of the Bay of San Fran- 

 cisco is the same as sea-water. The differences between Bay and 

 sea-water here are mainly in stillness, aeration, and organic con- 

 tent suspended and dissolved. The shore is heavy clay and be- 

 yond the marsh extend flats for greater or less distances, daily 

 covered by the tide. Into the marsh are cut tidal channels, here 

 called sloughs and pronounced slews! Larger or smaller fresh 

 water streams empty into the heads of these. Most of the 

 streams are dry in summer. The algae growing under these 

 conditions are correspondingly different in kind and habit from 

 those of the sea coast. 



On the Bay shore many shallow ponds have been made by 

 building sod-dykes and pumping the Bay water into them. 

 These ponds are filled in winter or spring (less frequently at other 

 times) by windmill or electric pumps and the water evaporates 

 during the summer. In a good season salt is crystalizing out 

 by late August. The water in the ponds changes, then, in about 

 six months from, say, a 3% solution of common salt to a saturated 

 solution. If there are late spring rains, the concentration of the 

 solutions in the ponds is less than 3% The algal vegetation of 

 these salt ponds is very interesting. Beginning with the spring 

 the vegetation of the ponds is similar to that on the marsh and 

 in the Bay As evaporation progresses there is an increase in 

 the unicellular motile forms, Carterias become abundant, and 

 the Ulvas, Enteromorphas, and Cladophoras decrease. Later 

 these and the Carterias disappear and their places are taken by 

 bicilliate Dunaliellas, green and red-brown, which serve as food 

 for the very peculiar crustaceans living in the increasingly saline 

 solution. Finally the color of the water in the ponds changes to 

 a red, in shade between blood and rust. This color, attributed 

 by various authors to iron salts or to the algae, is due, I believe, 

 to chromogenic bacteria. These, cultivated on brine to which 



