132 



BuivLETiN 79 



at this freezing point for some time, as will be noted late r, it 

 drops fairly regularly to the temperature of the air in the test tube, 

 which was kept usually at —20° C. (—4° F.). 



COMPARISON OF COOLING POINTS, FREEZING POINTS, AND DURATION 

 or FREEZING POINTS IN DIFFERENT SPINELESS CACTI 



1. Opuntia castillae. — The cooling point was reached at — 1° C. 

 (30.2° F.) and rose to —0.50° C. (31.1° F.), the so-called freezing 

 point, where it remained from 35 to 44 minutes, when the temperature 

 dropped for a while slowly, and afterwards rapidly, until ■ — 20°C. 

 ( — 4° F.) was reached. 



2. 0. ficus indica from Malta.- — The cooling point was reached 

 at - — ^.75° C. (30.65° F.). The temperature rose now quickly 

 to — 0.25° C. (31.55° F), the freezing point, where it remained from 

 12 to 18 minutes, then the temperature dropped quickly to ■ — 20° C 

 (—4° F.). 



3. 0. Ellisiana reached its cooling point at ■ — 0.75° C. (30.65° F.) 

 and then went up to — 0.50° C. (31.1° F.). Afterward it remained 

 at a temperature between —0.50° and —0.20° C . (31.1° and 31 .64° F.) 

 for about 20 to 28 minutes, when it began to drop rapidly. 



F ig. 6. — C-irvi showing cooling point, freezing point an;l duration of freezing point of 



Opuntia castillae. 



