92 CARNEGIE INSTITUTION OF WASHINGTON. 



Therefore, the total expected root-growth of Covillea during August 

 at Tucson would be 588.7 mm. as opposed to 74.4 nun. at Carrael. 

 By denominating the temperature of the soil as t and the hourly rate 

 as r, the biological significance of any summation of soil-temperatures, 

 tr, can be numerically presented, as in the foregoing table; and with 

 sufficient data on the temperature of the soil the tr could be plotted and 

 the ecological importance of the soil-temperature relation be graphically 

 expressed. An additional method of representing the relative efi^i- 

 ciency of different situations for any species consists in a comparison 

 of the tr for any temperature with the maximum tr for the stations. 

 For example, Carmel and Tucson may be chosen and the data as above 

 presented may be used. If every hour of the month of August were at 

 the optimum soil-temperature for Covillea, for example, the expected 

 root-growth would be 1,190 mm. It is thus to be seen that at Carmel 

 in August under actual conditions the expected root-growth of Covillea 

 can not be over one-sixteenth that at optimal soil-temperatures, 

 while at Tucson the total root-growth would be approximately half 

 that possible. Therefore it can be said that the soil-temperature at 

 a depth of 30 cm. at Tucson is approximately 8 times as effective for 

 root-growth of Covillea as the temperature of the soil at the same 

 depth at Carmel, a difference so great as to account for the survival 

 of the species in the one localitj^ and its failure in the other. 



Plant Distribution on Desert Mountains, by Forrest Shreve. 



In connection with the comparative study of physical conditions 

 on the isolated mountain ranges of southern Arizona, opportunities have 

 been made for a further examination of their vegetation, both with 

 respect to its character and its vertical limits. Data are also being 

 accumulated with an ultimate view to a comparison of the floras of the 

 forested portions of some of the large mountains. During the past 

 year visits have been made to the Santa Catalina, Pinaleno, Santa Rita, 

 Huachuca, and Whetstone mountains. Particular attention has been 

 given to detecting the plants which are conmion components of the 

 vegetation in the Pinaleno and Huachuca mountains but are not known 

 to occur in the Santa Catalinas. The former of these ranges is 1,500 

 feet higher than the Santa Catahnas and consequently presents a large 

 area in which the physical conditions are unlike those of any portion 

 of the latter mountain. The extensive forest of Picea engelmanni above 

 9,800 feet on the Pinaleno range and the small but well-developed 

 meadows and alpine parks afford habitats for a considerable number 

 of species which have not been found in the Santa Catalinas. At 

 lower elevations are also to be found a few species which do not 

 occur in the more isolated mountains, although appropriate altitudes 

 and similar conditions would appear to favor their invasion. The 

 Huachuca Mountains are well known to have a rich flora, comprising 



