244 BOTANICAL GAZETTE [OCTOBER 
connection. The loamy sand, constituting little floodplains on 
the margins of the actual streamway, is 30 cm. or more in depth 
and is underlaid by coarser material. This is the soil here con- 
sidered. It is practically without gravel admixture and has a 
water-holding power of about 25 per cent of its dry weight. It 
is almost certain (but without direct evidence) that an under- 
ground water table is present here, at least for many months of the 
year. The soil surface is nearly level and water is received from the 
superficial run-off of the hill and Larrea slope; however, it is well 
drained below. 
4. The soil mass of the river floodplain, now completely aban- 
doned by the stream, is of unknown depth. The surface layer is level 
and composed of clay loam with a water-holding power of about 
39 per cent of its dry weight. This is about 8-9 meters deep and 
rests on sands and gravels, permanent water being met with at a 
depth of 10-15 meters. This soil is characterized by general 
absence of surface run-off and by but slow percolation to lower 
layers. The dry soil of the deeper layers, however, removes Mol 
ture from near the surface as rapidly as it can move downward by 
capillary action. 
THE PLANT SOCIETIES.—The plant societies of the area about 
the Desert Laboratory have been thoroughly described by SPAL- 
DING in the work already mentioned, and also in a journal article 
(2). THoRNBER’s ecologically classified list of species for this regio 
(pp. 103-112 of the same monograph) is of especially great value 
to the student of environmental factors. The four types of vegeta 
tion which occupy respectively the four soil types here considered 
may be briefly characterized as follows: 
1. The Parkinsonia society of Tumamoc Hill possesses MOF 
forms, both woody and non-woody, than any other of the four ae 
cieties here considered. Besides this form of palo verde (Parkinson? 
microphylla), the group includes the giant cactus (Cereus giganteus); 
the ocotillo (Fouquieria splendens), the barrel cactus z 
nocactus Wislizeni), several opuntias of both cylindrical and ott 
stemmed types, Acacia constricta, Larrea tridentata, Celtis P allide, 
small specimens of mesquite, etc. Bigelovia Hartwegii occur 
here but does not attain as great numbers or as large size a5 OP the 
