194 THE PLANT WORLD 



tures. Ihe sugar maple in New York requires iioo hours 

 exposure to temperatures above the freezing point, receiving 

 in that time 7772 hour-degree-Fahrenheit units and being sub- 

 jecL to temperatures below freezing point for a number of 

 hours the total of which is approximately equal to that above. 

 The earlier plants of the sub-tropical desert about Tucs*3n 

 are therefore seen to require nearly three times as much Iieai 

 as the \'ernal forms of the temperate zone for the maturity 

 of the repn)ducti\e organs. The total exposures for the 

 year 1907 as measured at the Desert Laboratory are approx- 

 imately as follows : 



From winter solstice to February i, 21^514 hour-de- 

 gree-Fahrenheit units; from the winter solstice to the begin- 

 ning of the dry fore-summer, April i, 59071 hour-degree- 

 Fahrenheit units; to the beginning of the moist midsummer 

 160539 units; to the beginning of the dry after-summer, Sep- 

 tember 1, 236905 units. Total exposure for the year 1907, 

 32538 <;. Shortly before the winter solstice, the temperature 

 may fall below freezing point for four or live hours, thus 

 making a total of about 12 hours in the year during which 

 plants at the Desert Laboratory n^iay be subject to freezing 

 temperatures. 



Jl'iiiti'i- pt'it'iniidls. A large number of species with per- 

 ennial root stocks and bulbs push out new shoots, and form 

 leaves and flowers under the influence of the minimum ex- 

 posure noted abo\'e in conjunction with the abundant supply 

 of moisture. Among these may be mentioned the following: 

 Rrod'uu'ci ciipiUilii, ^^•ith a fe\\' blue or white Howers borne 

 on a slender scape with a deeply buried bulb; J in'iiioiic sphac- 

 iiop/iylld a relati\c of the crowfoot, with long-stalked cylin- 

 drical fruits; PciisIciudu ■:i-r!<^litii, with its crimson funnel- 

 shaped flowers, a succession of which follow through March 

 and April; and /\ purryi, on the slopes, with a metallic luster 

 to the slender corollas. Stalks of the last two species are 

 a favorite food of rock-squirrels, which cut them off near 

 the base and carry them away to their retreat. Later another 



