76 CARNEGIE INSTITUTION OF WASHINGTON. 



So far as biological phenomena and their interpretation are con- 

 cerned, the apparent equilibria may usually be treated as real ones, 

 because all important biological phenomena take place within a 

 limited time; it is only when an attempt is made to predict from 

 theoretical reasoning that caution must be observed in determining 

 the presence of true equilibria. 



Water Content and Abscission in the Nuts of Juglans californica querdna, by 



Francis E. Lloyd. 



Individuals of this mutant bore numerous fruits in the garden this 

 season. Usually three to five fruits were set on each peduncle, of which 

 ultimately all but one, if not all, were shed. This behavior is analogous 

 to the shedding of cotton bolls, of young oranges ("June drop"), etc., 

 which has been shown to result in the main from disturbed water- 

 relations in the plant. Through cooperation with Dr. D. T. MacDou- 

 gal, who was making auxographic studies of the Juglans fruits, it 

 was possible to obtain growth records of such as ultimately became 

 separated from the twig by abscission. 



An examination of a critical record shows that, previous to the 

 abscission period, there was a net gain in dimensions of the fruit on 

 each day, the amount of daily shrinkage being less than the amount 

 of growth. The net daily growth was sensibly less on each successive 

 day during the earlier period of observation of 8 days. For the sub- 

 sequent 3 days previous to separation, however, the relation of growth 

 to shrinkage was reversed, till on the final (eleventh) day, at the 

 conclusion of which abscission was complete, there was observable 

 only shrinkage, and this in marked amplitude. 



Observations on the abscission period showed that the process occu- 

 pies about 2 days in nuts as small as those measured auxographically. 

 It may be soundly argued, therefore, that the inadequate supply of 

 water to the growing nuts is the exciting cause of abscission. 



The method of abscission is similar to that described by the writer 

 for the cotton plant (Gossypium) . 



ECOLOGY. 



The Vegetation of an Imperfectly Drained Desert Valley, by Forrest Shreve. 



During the past year progress has been made in a study of the 

 physiography and vegetation of the Avra Valley, an imperfectly 

 drained desert basin lying west of Tucson. A drainage area of nearly 

 200 square miles is tributary to this valley, including the Baboquivari 

 Valley, a portion of Santa Rosa Valley, and several minor areas. 

 Although a number of intermittent waterways of considerable size 

 empty into the Avra Valley, there is no central and permanent drainage 

 from it into the Santa Cruz River, toward which the valley stretches 

 with a falling gradient. 



