Cold-Resistance in Spineless Cacti 131 



Slides of tissue of cactus plants were put for different periods 

 on ice, but the cells were not damaged. This suggests that the 

 cells are not injured with the temperature of melting ice. The 

 first problem is, therefore, disposed of. 



Protoplasmic Movements 



It is interesting to observe the movements of the protoplasm of 

 the living cdls of cactus stems under different temperatures. In 

 studying a tissue at room temperature, which was about 29° C, 

 (84.2° F.), the streaming of the protoplasm is noticeable through 

 the rapid movements of the microsomes and physodes; th^^ large 

 chloroplasts also, and not infrequently the nucleus, are often 

 seen to move for a short time. When the temperature is gradually 

 lowered by putting ice arDund the slide, the protoplasmic movement 

 becomes slower and slower until it finally stops. The location of 

 the different protoplasmic structures and the size of the cells, how- 

 ever,remain the same ; and the cells have not lost their turgor. After 

 the ice has melted, the water around the slide soon takes the tem- 

 perature of the surrounding air and the protoplasm in the cells 

 again begins its movements. 



determination of the cooling and the freezing points in cacti 



The second problem is very interesting and raises several addi- 

 tional questions. It takes into consideration the behavior of the plant 

 at temperatures below the freezing point. The following experi- 

 ment was performed in this connection. Pieces about 10 by 10 by 

 25 millimeters in size were cut from the stems of cactus plants. In 

 the middle of these the bulb of a thermometer was inserted. The 

 thermometer, with the piece of cactus in position, was next placed 

 in a large test tube and the opening closed with a plug of cotton 

 batting. This test tube was next placed in a mixture of ice and 

 salt so that a temperature of —20° C. (—4° F.) was reached. 



The thermometer soon dropped from the temperature of the 

 cactus to —1° or —0.75° C. (30.2 or 30.6° F.) the cooling point, and 

 then rose quickly to 0.25° to 0.50° C. (32.45° to 32.9° F.). After 

 this the temperature remained for a considerable time at 0.50° to 

 — 1.50° C. (32.9° to 29.3° F.). This latter temperature is known as 

 the freezing point and has a remarkably small variation with the 

 different species of cacti that were studied. The length of time, 

 however, during which this temperature is maintained by the different 

 species of cacti varies considerably. After the temperature remains 



