DESERT PLANTS 549 



iu 1908. An afternoon in October, 1909, was spent in felling and 

 cutting up a tree cactus (Carnegiea or Cerus giganteus), near the Desert 

 Laboratory, which consisted of a single cylindrical trunk 18 feet in 

 height. The total weight was nearly a ton, and a section was found to 

 contain over 91 per cent, of water, showing that the entire plant held 

 over seventeen hundred pounds of water, or about five barrels. 



It has previously been pointed out that during the dry season these 

 plants sustain only an anchorage relation with the soil, and that absorp- 

 tion ceases wholly. The experiments were therefore planned to detach 

 a number of individuals of the sahuaro {Carnegiea), the melon cactus 

 (E 'chino cactus) , and various opuntias from the soil, place them on 

 suitable supports in the accustomed upright position and thus simply 

 lengthen the dry seasons to which they had been subject. Accidents in 

 nature tear man}^ individuals loose from the soil and they may not be 

 able to perfect a new root-system for many months, so that the observa- 

 tions closely simulated happenings in the history of the species involved. 

 Some of the test plants were placed in the open air, some in the more 

 equable conditions of a well lighted laboratory room, a few were kept for 

 periods of a few months in constant temperature dark room, and others 

 were exposed to the full blaze of the Arizona sunlight, standing on a base 

 of black volcanic rock, thereby avoiding none of the desiccating effects 

 of the climate. 



The formidable armature of the bulky bodies of these plants made 

 their manipulation a matter of some difficulty and discomfort even 

 with the best supports and harness that could be devised. The larger 

 ones were placed on platform scales, where they were allowed to remain 

 undisturbed. The majority, however, were mounted and two or three 

 men were necessary to handle them in the weighings which were made 

 at intervals correlated with the season and the rate of loss. 



All individuals showed a high rate of loss when first taken from the 

 soil, the excess being attributed to the evaporation from abraded sur- 

 faces of the roots and stems. Next it was found, of course, that the rate 

 of loss was least during the cooler season, at which time an E chino cactus 

 might lose as little as one forty-thousandth of its weight in a day, and 

 on the other hand during many days in the hot dry season the daily loss 

 was one three-hundredth of its total weight. The minimum of the tree 

 cactus was one nine-thousandth of its total and the maximum was about 

 that of the Echinocactus, although not measured under equivalent con- 

 ditions. 



Chief interest in the rate of loss, however, centers about the behavior 

 of these plants from season to season, especially when the amount of 

 water on hand was taken into account. In work of this kind it is found 

 convenient to use a standard of succulency which calculates the number 

 of c.c. of water to 100 sq. cm. of surface. Thus a great melon cactus 



