No. 469] 



BIOLOGY OF CACTI 



41 



Cereus per hour was approximately 33 milligrams while that of 

 Echinocactus on September 3 was 9.G milligrams. Whether 

 this difference in the rate is constant for the two genera, or is 

 attributable to other and unknown causes aside from the rela- 

 tively slight difference in surface, is not known. 



A specimen of Opunfia v( rsicolor abont 20 cm. high, growing 

 near the laboratory buildiiiu-. was studied at various times during 

 the dry portions of the year ;I!HK.. narnrly, in March, April, 



such for instance as Covillea tridentaia, KnccUa jarinosa, or Fou- 

 quieria splendens (Cannon, :05a, p. 404), suggesting a unique 

 position among its associates and an important factor among 

 the varied ones that brought about the present distribution of the 

 group to which it belongs. 



As has b(>en shown in another place, the transpiration of Fom- 

 quieria splendens, as well as that of other desert plants and plants 

 of more humid regions (Burgerstein, : 04), under certain con- 

 ditions increases with an addition to the available water supply 

 (see Cannon, :05b; V. M. Spalding, : 04, :05). Thus after 

 rains, but before leaves appeared, the rate of transpiration of 

 Fouquieria splendens increased about three fold; after leaves 

 had been formed and while they were developing the rate was 

 relatively very great. A similar condition was likewise observed 

 in Encclia fnrinosa, Covillea fridenfafa. and in other ])lants. In 



other Opuntias the response to the rains is indeed llso positive 



and as a consequence it at once inen-ascs in si/e. and its ti--ues 

 become turgid. But, so far as I oh-erved. the rate of tran-^pi ra- 

 tion did not increase proportionally. In.lee<l, hiboratorv c.\j)eri- 

 ments, in which a small specimen of O. rt rsiculnr was attached 

 to a potometer by a long delicate tube so that the cactus could 

 be weighed at intervals at the same time that its rate of absorp- 

 tion was being recorded, showed very clearly that under such 



