68 THE RELATION OP DESERT PLANTS TO 



regards the relation of weight to volume in this case, 45.25 grams of dry 

 soil occupied, when allowed to settle under water, 43 cc. 



These data are in very good agreement with those obtained for cab- 

 bage and squash at Tucson, and probably approximate an average 

 wilting point for most mesophytes. 



Of all the determinations made for the wilting point of desert plants 

 only two soils surpassed 10 per cent in water content, these being in 

 the cases of Experiments VIII and XL It was pointed out on page 11 

 that the soil of Tumamoc Hill at the end of the spring dry season 

 contained from 5 to 10 per cent of moisture at a depth of only 10 or 12 

 cm., while, as far as evidence is at hand, it appears that from 12 to 15 

 per cent of moisture occurred at depths not exceeding 40 cm. Thus it 

 is seen that even in the driest part of the year the moisture content of 

 the soil at a depth of not over 30 or 40 cm. is probably high enough to 

 readily supply such plants as Fouquieria, Euphorbia, Tribulus, and 

 even, perhaps, Allionia and Boerhavia, with transpiration water. As 

 far as the first three forms are concerned (Allionia and Boerhavia 

 are not commonly seen here excepting in the rainy season), it seems 

 that these soils are not excessively dry even at the end of the dry sea- 

 son. Such plants as Brassica, Cucurbita, Vicia, Phaseolus, and Helian- 

 thus must succumb to drought conditions somewhat sooner. It was 

 observed in growing the seedlings of squash and cabbage that they 

 required watering several times a day in order to keep them in health, 

 while Euphorbia and Fouquieria could not only live, but thrive for many 

 days, in a similar vessel of the same soil without watering. 



The roots of seedling Fouquierias were often observed to have pene- 

 trated to a depth of 10 cm. or more within 48 hours after the first ap- 

 pearance of the cotyledons,* and when it is remembered that the deeper 

 layers of this soil dry out very slowly after being wet by rain, it is 

 easily seen how such seedlings, germinating in the rainy reason, may 

 attain to a depth where they will have a permanent and adequate water 

 supply before the upper layers of the soil have dried out sufficiently to 

 produce death. Seedlings of Boerhavia and Tribulus are also very 

 active in the elongation of their primary roots, and all of the desert 

 plants studied were characterized by very long tap roots without lateral 

 branches. Although Boerhavia thrives only in the rainy season, it was 

 found impossible to lift seedlings of this form more than two or three 

 days after the cotyledons appeared without cutting off their roots. 

 These organs penetrate into the crevices between the rock fragments, 

 so that it is extremely difficult to remove them to a depth greater than 

 from 15 to 30 cm. It appears that plants whose habitats are in the more 



*Covillea has the same habit in germination as has Fouquieria when the soil is 

 rather dry. See Spalding's figure 3, in the paper (1904) already cited. 



