December 17, 1915] 



SCIENCE 



877 



upon south slopes and summits, the latter upon 

 gentle north slopes; and the Oak-Madrona Forest 

 on steep north slopes and in ravine bottoms. 



Measurements of evaporation, soil moisture, and 

 soil temperature were made covering a period of 

 eighteen months to determine the habitat differences 

 to which these vegetational differences are due. It 

 was found that the evaporation rate was greatest 

 on summits and south slopes and least on north 

 slopes, as would be expected, but that at the top 

 of the vegetation the differences in evaporation 

 between the aspects were slight. Greater differ- 

 ences were found at the surface of the ground. As 

 to soil moisture, a great difference was found be- 

 tween the two slopes during the rainy season, but 

 this difference gradually decreased during the dry 

 season, almost disappearing at its close. 



The conclusion of the study is that the actual 

 physical habitat differences between north and 

 south slopes, though perceptible, are slight, and 

 that evaporation is the fundamental factor. The 

 striking differences seen in the evaporation rates 

 at the surface of the ground, in the soil moisture 

 on the two slopes and at different times of the 

 year, and in the soil temperatures, are due in very 

 large part to the reaction of the vegetation upon 

 the habitat. 



Discussion: Abrams and Clements. 



Distribution of Cacti with Reference to the B6le 

 Played ty Soot Eesponse: W. A. Cannon, De- 

 partment of Botanical Eeseareh, Carnegie Insti- 

 tution of Washington. 



In southern Arizona the roots of the cacti lie 

 relatively close to the surface of the soil and are 

 subject to the maximum temperature changes, in- 

 cluding the highest temperatures of the summer 

 season. Experiments show that a relatively high 

 temperature of the soil is necessary to the best 

 root growth of the cacti. Owing to the arid fore- 

 summer in southern Arizona such root growth takes 

 place in midsummer only, when the seasonal rains 

 come. Since active growth does not occur in the 

 colder portions of the year, although the soil may 

 be moist, the possibility is suggested that the cacti 

 as a family are mainly limited to such regions as 

 have summer rains, other conditions being favor- 

 able. 



A comparison of the climatic conditions of those 

 portions of America, where the cacti form a prom- 

 inent portion of the flora, shows that as a matter 

 of fact, they agree in this, that rains are charac- 

 teristic of the warm season. On the other hand, 

 in regions otherwise apparently favorable, the 

 cacti are either wholly wanting, or they constitute 

 only an insignificant part of the flora. 



Climatic Cycles and Succession: F. E. Clements, 



University of Minnesota. 



Discussion: Abrams, Cooper, Eastwood. 

 The Diversity of Ecologic Conditions and Its In- 

 fluence on the Richness of Floras: John W. 



Habshbeegee, University of Pennsylvania. 



(Read by H. M. Hall.) 



Ecologic conditions are those which are associ- 

 ated with the environment. They include the in- 

 fluence of climate, soil, physiography, chronology 

 and the life relations of the surroundiugs. The 

 influence of these conditions on the richness of 

 floras may be considered statistically. The gen- 

 eric coefficient, which is the relativity of genera 

 and species, is inversely proportional to the diver- 

 sity of the ecologic conditions. The generic co- 

 efficient was worked out for the floras of Point 

 Pelee, Ontario; for the pine-barrens of New Jer- 

 sey; for Hartsville, South Carolina; for the Alta- 

 maha Grit Eegion of Georgia; for Miami, Flor- 

 ida; for the Florida Keys; for the Upper Susque- 

 hanna, Pennsylvania; for Lancaster County, 

 Pennsylvania; for Columbia, Missouri; for Jack- 

 son County, Missouri; for the Tosemite National 

 Park, California; for the state of Connecticut; 

 for the state of Pennsylvania; for Alabama; for 

 the central Rocky Mountains; for the state of 

 Washington. It was found that Point Pelee with 

 a simple topograph was at one extreme with a 

 generic coefficient of 74.7 per cent, and the Central 

 Rocky Mountains and the southeastern United 

 States at the other extreme with highly diversified 

 and generic coefficients of 23.9 per cent, and 23 

 per cent, respectively. 



Discussion: Clements, Hall, Eastwood. 

 Plant Succession in the Palo Alto Region: W. S. 



COOPEE, Stanford University. 



The Palo Alto quadrangle comprises two dis- 

 tinct physiographic areas: (1) the mountains, cov- 

 ered mainly with residual soils, and (2) the alluvial 

 slope to the bay and beneath it, composed entirely 

 of transported soils derived from the mountains. 



On the latter area there is a very perfect suc- 

 cession of vegetational stages, correlated with the 

 building up of the alluvial slope by stream depo- 

 sition. The stages are as follows: (1) Algse, in 

 the shallow water of the bay; (2) Salt Marsh; 

 (3) Composite-Willow Formation; (4) Oak For- 

 est, composed mainly of Quercus loiata and 

 Quercus agrifolia; (5) Chaparral, mainly Ade- 

 nostoma. The Salt Marsh replaces the Algee soon 

 after the soil surface emerges at low tide. The 

 Composite-Willow formation follows with the elim- 

 ination of the salt from the soil. The Oak Forest 

 appears when there is sufficient feeding ground 



