354 BOTANICAL GAZETTE [NOVEMBER 
the plant body follows very closely upon that of the surroundings, 
and that soil temperature and air temperature are, roughly speaking, 
somewhat closely related. Furthermore, the heat condition of 
plants, as approximately measured by the temperature of the 
surrounding air, varies only in respect to intensity and duration; 
qualitative fluctuations are not met with here. Geographically, 
the present study deals with the area occupied by the United States. 
Direct temperature summation 
The effectiveness of temperature conditions to promote plant 
growth in any locality has been measured by phenologists,$ by means 
of the direct summation of the daily mean temperatures, such 
summation extending through the period of any particular phase of 
plant development which might be considered; for example, the 
period extending from the time of germination to that of flower- 
ing or of seed-maturation. In these temperature summations @ — 
certain minimum temperature is assumed as a starting-point, and 
the amount added to the summation for each day as the season 
advances is the number of degrees, above the assumed minimum, 
which represents the mean temperature for that day. The mini- 
mum has sometimes been o° C., more often a somewhat higher 
temperature. In the employment of such temperature summations, 
each station of observation is characterized each year by its sum- 
mation index, and after a period of years these indices may be 
averaged to give a measure of the temperature factor in general, 
for that particular place. : 
n a way somewhat similar to that followed by workers in 
phenology, Merrram® has obtained normal summation indices of 
temperature for a large number of stations in the United States, 
and has presented these in the form of a chart showing climatic 
zones, each characterized by its own range of temperature summa 
5 In this connection see ABBE, Joc. cit. 
° Merrtram, C. H., Laws of temperature control of the geographic distribution of 
animals and plants. Nat. Geog. Mag. 6:229-238. 1894. The same work was ag 
reported, in still more abbreviated form, in part III of the following paper: Mexais™ 
C. H., Life zones and crop zones of the United States. U.S. Dept. Agric., Div. Biol. 
Survey, Bull. ro. 1898. 

