238 



The Plant World. 



cases, to fall two pentads from where they belong. The rapid 

 and consistent fall of the curve, however, makes it appear prob- 

 able that the errors of the method have largely neutralized 

 themselves. The lack of germinations during the last three 

 pentads may be partly due to the overlooking of small individ- 

 uals, but those which are ten years old should be about 18 cm. 

 high and those 15 years old nearly 30 cm. high, which would 

 have made it difificult to overlook them. 



This curve as drawn is actually an age curve for the sur- 

 viving population, and falls short of being the curve of the rate of 

 establishment by just so far as there have been deaths among 

 the juvenile population during the 60 years since 1850. Deaths 



1850-55 1655-60 1860-65 1665-70 1870-75 1«" '" 1880-85 ie85-90 189O-05 1895-1900 1900-05 1905-10 



Figure 1. 



among the juvenile and adult cacti, just as with the seedlings, 

 are due in at least 90% of cases to mechanical factors, — erosion, 

 wind, lightning, etc., followed by the work of bacteria and 

 fungi. The deaths of the last six or eight years on the area over 

 which the census was taken, as indicated by fallen skeletons and 

 stumps, were all among the older two-thirds of the population, 

 but during the sixty years there must have been some deaths 

 among the juvenile third, which if known would increase the 

 steepness of the curve, making it a true curve of the rate of es- 

 tablishment. 



[ The probability seemed strong that the fall in the rate of es- 

 tablishment of the Giant Cactus on the north slopes of Tumamoc 

 Hill is related to the fact that it is always more abundant on 

 southern than on northern slopes in the vicinity of Tucson. To 

 carry the method used to other localities raised the objection 

 that the growth data used as a basis were secured on individuals 

 growing on the north slope of Tumamoc Hill, and were not ap- 



