The Rate of Establishment of Giant Cactus. 239 



plicable to cacti growing elsewhere. A few data as to the growth 

 of adults on the mesa in the vicinity of Tucson, obtained by the 

 photographic method, show a yearly rate of 7 cm. as against 15 

 cm. on Tumamoc Hill. So great a difference is entirely in accord 

 with the higher average water-content of the soil of Tumamoc 

 Hill as compared with the mesa, a difference which is accompan- 

 ied by profound differences in the character of the vegetation. 

 However, in the absence of anv other growth data, those of Mrs. 

 Spalding have been used, with an error W'hich finds ex- 

 pression in a shortening of the x-coordinate of the curve, so as 

 to make the rate of establishment which has been going on for 

 100 to 120 years appear as that of the last 60 years, without pro- 

 ducing any alteration in the character and significance of the 

 curve. 



i»?e-55 1855-40 1860-65 1865-70 1870-75 1675-80 1680-85 1885-90 1890-95 1895-1900 1900-05 1905.10 » 



Figure 2 



In order to secure a census of an area with southern exposure 

 a steep slope at the base of the Santa Catalina mountains was 

 chosen, w^here the population is very dense, the younger individ- 

 uals being conspicuously numerous and the old much-branched 

 ones infrequent. The growth rate in this situation is un- 

 doubtedly lower than on Tumamoc Hill and probably as low as 

 on the mesa near Tucson. A census of 175 plants was made, 

 of which 64% were less than 5 m. in height, — or less than 60 

 years old. Plotting the ages of the juveniles, as before, the curve 

 shown in Fig. 2 was obtained. In the area covered by this cen- 

 sus there was very little small vegetation to hide seedlings and 

 small juveniles, yet the curve shows the same fall during the last 

 eight pentads. The fact that there is a rise in the curve during 



