40 THE RELATION OF DESERT PLANTS TO 



accompanied by marked hardening of the tissues and by the formation 

 of a true bark. In the meantime the primary root grows directly 

 downward without branching, probably attaining a length in the open 

 soil of many decimeters. In the seedlings grown in pots the roots 

 extended around the base of the pot and finally branched profusely in 

 their distal portions. Not until the root has obtained a remarkable 

 length and the hypocotyl has become enormously thickened and very 

 woody, does elongation of this organ begin again. The plumule, which 

 has been dormant up to this time, then begins slowly to elongate, the 

 first true leaves being produced as much as a full month after the first 

 appearance of the cotyledons. 



Several cultivated plants of the more humid regions, such as squash, 

 beans, etc. , were also grown from the seed and used for purposes of 

 comparison. 



Growth of all these forms, excepting the aerial portions of Fouquieria, 

 was exceedingly rapid at this season of the year. The Boerhavia and 

 Tribulus plants were in full bloom within four or five weeks after their 

 cotyledons appeared. This, it is to be remembered, was during the 

 hottest season. The high temperatures which prevailed seemed to 

 have no deleterious effect upon any of the native plants, nor upon the 

 cultivated plants experimented with, so long as an ample supply of 

 water was provided for the roots, thus allowing the excessively high 

 transpiration to be kept up. 



WATER REQUIREMENT FOR GERMINATION. 



Seeds of Fouquieria splendens were planted in Stender dishes con- 

 taining soil of several different water contents and note made of their 

 germination. In soils containing 5 and 10 per cent of water by volume 

 the seeds failed to germinate. In the latter soil the wings and outer 

 layers of the seed coat softened and became somewhat like moist paper, 

 but in the former such signs of absorption were hardly perceptible. 

 In a soil containing 15 per cent of water the seeds germinated at last, 

 although germination occurred much sooner in the 20 per cent sample. 

 They germinated earlier in moister soils up to 40 per cent, but were 

 soon destroyed by fungi in 30 per cent and above. It thus becomes 

 evident that, at the temperatures of the summer rainy season, Fouquieria 

 seeds require for germination a moisture content in the soil of about 15 

 per cent, while they germinate and develop well in soils of higher 

 moisture content up to about 25 per cent. 



Seeds of Cereus giganteus were found to germinate well in 15 per 

 cent soil and with higher moisture content, but soon died with apparent 



