Volume 13 Number 10 



The Plant World 



A Magazine of General Botany 

 OCTOBER, 1910 



THE RATE OF ESTABLISHMENT OF THE GIANT 



CACTUS. 

 Bv Forrest Shreve. 



The characteristic openness of desert vegetation affords am- 

 ple room for the establishment of new individuals of the perennial 

 species, a marked contrast to what is true of forests or grassland. 

 The number of germinations that take place during any rainy- 

 season in the desert is very great as compared with the number of 

 estabUshments that result from them. The low water-content 

 of the soil, — which is by far the most important factor in pre- 

 venting establishment,— most frequently operates to kill seedlings 

 soon after the close of the rainy period in which they genninated, 

 or, in a smaller number of cases, becomes operative after several 

 years of successful growth have brought the root systems into 

 regions of the soil that are already being drained by perennials 

 that have been established for a long time. The rainy seasons 

 of the southwestern deserts vary from year to year in the amount 

 of rainfall and in their duration, liuctuations which have much. 

 to do with the persistence or death of the seedhngs. 



The widely diverse types of desert perennials differ greatly 

 in their behavior as respects gern ination under natural condi- 

 tions and as respects the stage in their early life at which the 

 factors that inhibit establishment become operative. In gen- 

 eral the schlerophyllous shrubs have numerous genninations, 

 with a very high death rate at the end of the first growing season 

 and a rather high one in the second and third years. The suc- 

 culent perennials, — at least the cacti, — have few germinations, 

 but the percentage of survivals among them is very high, not 

 only in the first but in the succeeding years. The deeply rooted 



