Deserts 



strange modification of the protoplasm which enables 

 them to resist heat, and also to breathe internally without 

 opening their pores, which might allow the dangerously 

 dry air to enter their bodies. 



In the American desert a thermometer placed in the 

 body of a cactus (Cereus sp.) showed a temperature of 

 45 to 46 C. Indeed their body temperature is often 

 above that of the earth near them. Askenasy found 

 that when the sand showed only 43 to 44 C, the leaf 

 of a Sempervivum had a temperature of over 51 C. 

 In one day they are subject to extraordinary fluctua- 

 tions. At 7 to 8 A.M. the leaf of Sempervivum tectorum 

 showed 1 3 C, and at 2 to 3 p.m. it was at 49. 6° C, 

 which means a difference of over 33 in a single day. 



Their exact system of keeping alive in deserts is not 

 understood. In dry weather Cereus and some other 

 cacti seem to shrink together so as to show deep longi- 

 tudinal grooves. This shrinking will close up their 

 stomata or breathing spores. 3 



In wet or misty weather they expand so that the 

 grooves become shallower, like those of a camera-case 

 when drawn out. Then the moist air penetrates into 

 a curious system of air channels, which branch in and 

 out amongst the green assimilating cells. At such 

 times they are actively growing and even forming 

 (Echinocactus sp.) green leaves, which drop off as soon 

 as the drought begins. 4 



They have the curious property of being able to give 

 out carbonic acid without taking in oxygen. One does 

 not like to charge them with alcoholism, but it seems 

 that they must somehow produce oxygen for respira- 

 tion without allowing the dry outside air to enter the 

 body. So it is quite possible that some kind of alcoholic 

 fermentation is really going on within these innocent- 

 looking fleshy stems. 



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