236 The Plant World. 



seedlings of the sclerophyllous types which survive the first 

 dry period after germination are almost exclusively killed only 

 by low and prolonged depressions in the annual curve of soil 

 moisture, while the relatively shallow-rooted seedlings of the 

 cacti are much more liable to mechanical injury or destruction 

 by the erosion accompanying heavy rains, by the impact of se- 

 vere rainfall, by the burrowing and scratching of rodents, or 

 by being eaten by rodents in search of moist food. 



In order to determine the rate of establishment of one of 

 the most characteristic desert succulents, the following statisti- 

 cal method was devised and applied to the Giant Cactus (Carne- 

 giea gigantea, Cereus giganteus) . By reason of their simple form 

 the young, unbranched Giant Cacti may be measured easily 

 and their rate of growth determined with accuracy. Mrs. E. 

 S. Spalding carried out a set of such measurements for several 

 years and has been good enough to permit the use of her figures 

 in advance of their prospective publication. The sixteen indi- 

 viduals measured by Mrs. Spalding were of all sizes from 1 to 

 15 dm., and some of them were followed through four years. 

 Her data have been supplemented by measurements on two 

 lots of seedlings of known age and by computation of the rate of 

 in a few individuals over 15 dm. high. Young plants less than 

 1 dm. in height are so rare, or inconspicuous, that nine botanists 

 who have had excellent opportunities to find them report that 

 they have never done so, and there is accordingly some doubt 

 as to the growth rate during the earliest years under natural 

 conditions. The assumption that individuals 1 dm. in height are 

 eight vears old is based partly on the direction of the growth 

 curve to be mentioned, and partly on the growth rate of two 

 lots of seedlings, one and two years old, which (unfortunately 

 for this purpose) have been grown under partial irrigation. The 

 growth rate of the individuals over 15 dm. in height was obtained 

 by comparing old photographs of certain individuals in the vi- 

 cinity of the Desert Laboratory with their present size. The 

 conspicuous forkings of the ridges common to all Giant Cacti, 

 served to give measurements from fork .to fork, and from the 

 uppermost fork to the tip, that supplied reasonably accurate 

 data, showing an average rate of 15 cm. per year. 



With the growth data in hand, a curve of growth was se- 

 cured by plotting rates of growth in cm. per year with heights 



