294 The Plant World. 



inclined, then, to consider the figures just given as showing 

 nothing with regard to a rise or fall of the establishment rate 

 from 1490 to 1840, since the rise in number of individuals dur- 

 ing that period merely indicates a shortening of the period over 

 which various causes of death might overtake the trees. Th*^. 

 fall in number of survivors in the last three periods, however, is 

 not influenced by this erroi, oi lather if the error is effective 

 it has merely served to make the fall in that pait of the curve 

 less steep. The figures of the last three periods indicate, then, 

 that there has been a fall in the rate of establishment during the 

 last 60 years. That this fall lies over the same period in which 

 I found a decrease in the establishment rate of the Giant Cactus 

 indicates, tentatively, that whatever secular changes of climate 

 may be taking place are affecting the largest of the succulents 

 and the largest of the sclerophyllous trees in the same manner. 

 My detailed observations of the present rate of germination 

 and seedling mortality have been made over an area of 640 sq. 

 m., lying within the area over which the above mentioned census 

 was taken. My method has merely been to go over the area 

 carefully every few da}s at the time of the first summer rains, 

 and to drive a numbered stake near each seedling. No germi- 

 nations take place at any other time of the year. At periods of 

 every two or three weeks during the year I go over the area 

 and remove the stakes from all seedlings that have died since the 

 last visit. The summer of 1910 was a very favorable one for Palo 

 Verde germinatic ns, and the total number was 542, over an area on 

 which there are only eight adult trees living at the present time. 

 Without giving the detailed record of the annual march of soil 

 moisture and evaporation conditions, which 1 hope to do latter 

 in a more detailed treatment of this entire matter, I will merely 

 refer the number of deaths in the seedling crop of 1910 to the 

 somewhat unique seasons of southern Arizona in the following table, 



SEASC>N NUMBER OF NUMBER OF 



DEATHS SURVIVORS 



Humid Mid-Summer, 1910 43 499 



Arid After-Summer, 1910 157 342 



Winter, 1910-11 74 268 



Arid Fore-Summer, 1911 146 122 



Humid Mid-Summer, 1911 60 62 



