94 CARNEGIE INSTITUTION OF WASHINGTON. 



per annum under the most favorable conditions. No germinations of 

 the barrel cactus (Echinocactus wislizeni) have been observed on the 

 area, and only one seedling was found on the establishment of observa- 

 tions. During the past four years this seedling has grown 70 mm. in 

 height and 60 mm. in diameter, reaching a height of 110 mm. The 

 evidence with respect to these two massive succulents, both on this 

 observational area and on other areas that have been less thoroughly 

 studied, is that they produce very few seedlings and that a relatively 

 high percentage of them survive and reach mature size. 



The number of germinations of palo verde (ParJdnsomo micwphylla) 

 has been extremely variable, ranging from none in 1915 to 542 in 1910. 

 This is due to the fact that seed crops are borne only in very favorable 

 years and that the waxy coating of the fresh seed delays its germination 

 from one to several years. The early years of work on this area demon- 

 sti'ated that the rate of mortality of the seedlings of palo verde is deter- 

 mined by soil-moisture conditions and not by competition with each 

 other nor with other vegetation. Germinations take place only in the 

 summer rainy period, and the number of deaths among the seedlings 

 of a given year is very great during the arid season immediately fol- 

 lowing. The mortality continues to be high for two or three years, 

 at the end of which time the population has usually been reduced to a 

 low percentage of the original number of germinations. In 1917 there 

 survive only 2 of the 542 individuals v^^hich germinated in 1910, and 

 only 5 of the 29 that germinated in 1916. The total number of 

 germinations in the seven summers of observation has been 1,198, and 

 the number of survivals is 19. 



The seeds of ocotillo (Fouquieria splendens) are produced in great 

 profusion and the number of seedlings that appear with the first rains 

 of summer is so great that it is impracticable to keep a record of them. 

 The arid after-summer causes the death of all but an extremely small 

 number of these seedlings. The record of the survivors has been begun 

 in their second year. The total number of ocotillo seedlings surviving 

 into the second summer has been 21, while 5 were found on the com- 

 mencement of observations. In the summer of 1917 there were 7 of 

 these 26 surviving, which is probably a representation of 1 for each 

 50,000 to 100,000 germinations for the seven years. 



The extremely slow rates of growth of desert perennials have been 

 famiUar for some tim.e. It appears from the observations just de- 

 scribed that the establishment of new individuals in the plant popu- 

 lations of the desert is also an extremely slow process, even for plants 

 which are as abundant and characteristic as those that have been used. 

 The low rate of establishment, slowness of growth, and attainment of 

 great longe\dty combine to give desert vegetation a conservatism of 

 change which is in keeping with the adverse conditions under which it 

 lives and is in sharp contrast to the very rapid development and 



