1920] Allen: Plankton of the San Joaquin River 15 



Jussiaea calif arnica fills some quiet sloughs and canals with an al- 

 most impenetrable mass of stems. 



Scirpus lacustris L., is by far the most conspicuous plant in the 

 marshes and shallow waters from one end of the valley to the other. 



Carex marcida Boott. covers large areas of ground where the soil 

 remains saturated though not completely submerged through most of 

 the year. 



Anemopsis californica Hook, occurs in temporary marshes and is 

 peculiar to the rainy season. 



The most abundant willows are Salix nigra Marsh., S. lasiolepis 

 Benth., and S. fluviatilis Nutt. Although abundant by natural propa- 

 gation, they are frequently planted along the levees to help to hold the 

 dirt in place. 



Popnlus fremont'i Wats, is common along the water courses, but 

 not abundant. 



Ruinex salicifolius Wats, is conspicuous in marshy ground, espec- 

 ially in the heavy loam and peat soils. R. occidentalis Wats, and R. 

 Crispus are also prominent. 



Polygomini amphihium L. occurs in the ditches and narrow water- 

 ways. So far as observed, it furnishes remarkably good shelter for 

 minute animals and plants. 



Ranunculus aquatilis L. is exceedingly abundant in small areas at 

 times, forming dense mats in shallow water. 



Nasturtium officinalis is reported as common, but it has not been 

 observed by the present writer. 



THE COLLECTING STATIONS 



Three collecting stations were used. Plate 20 Station I was located 

 in Stockton Channel at the foot of Yosemite street. This is about one 

 mile and three-quarters from the river ; four hundred yards from 

 Mormon Channel outlet and three-fourths of a mile from the steamer 

 landing at the head of Stockton Channel. There was a good deal of 

 sewage coming down this channel. Mormon Channel was an open 

 cesspool during most of the year. Hence this station seemed to be 

 fairly typical for the study of organisms in dilute sewage. 



Station II was in the river, from four hundred to eight hundred 

 yards above Stockton Channel, and it represented as nearly natural 

 river conditions as could be found in this section. 



Station III was in Smith Canal about four hundred yards from 

 the river. There was a small amount of sewage coming down this 



