144 The Plant World. 



field at appropriate times. The three experiments in which 

 individuals were given from 29 to 46 hours of freezing redupli- 

 cated the conditions to which they would be subjected at higher 

 altitudes and to the northward of their natural range, and they 

 indicate the inability of Carnegiea to withstand such longer 

 durations of cold, 1 made no examination of the tissues of 

 the cacti which were given the longer durations of frost. It is 



probable that 20 to 36 hours of frost would kill some individuals 

 and would fail to kill others, and it is only the jump in the con- 

 trolling climatic factor, to which allusion has been made, that 

 fnakes the natural ojeration of this factor decisive and limits 

 the distribution of Carnegiea in an abrupt manner. The oc- 

 currence of a single day without mid-day thawing, coupled with 

 a cloudiness that would prevent the internal temperature of 

 the cactus from, going above that of the air, would spell the de- 

 struction of Carnegiea; and the parallel evidence of the climato- 

 logical records and of the experiments which have been described 

 appears to explain the limitation of its northward distribution. 

 The branched arborescent cactus Opuniia versicolor ascends 

 in the desert mountain ranges to v5,800 feet (1,768 m.) and the 

 small form Echinocereus polyacanihus reaches 7,800 feet (2, i80 

 m.), the highest altitude attained by any species of cactus in 

 southern Arizona. Opuniia versicolor is common on the desert 

 mesas and reaches its highest limit only on ridges and south- 

 ern slopes, while the highest individuals of Echinocereus polyacan- 

 ihus are found on cliffs and rocks where they are likely to be 

 exposed to insolation and mid-day thawing. Freezing experi- 

 ments similar to those described were carried out on young 

 individuals of Opuniia versicolor 5 to 25 cm. high and on adults 

 of Echinocereus polyacanihus. The 1 1 individuals of these 

 species used were given various lengths of exposure up to 66 

 continuous hours of freezing without any injuries or fatalities. 

 The longest of these exposures is about the length of time that 

 frost would supervene when two consecutive days were without 

 thaw, a condition which must exist in the mountains of Arizona 

 at 7,1^00 feet altitude in every severe winter. The smaller and 

 more widely distributed types of cacti nmst all be more resist- 

 ant to frost than are any of the arborescent colunmar or the 

 branched arborescent types. In the Rocky Mountains 



