SOIL MOISTURE AND TO EVAPORATION. 41 



damping off in soils of 25 per cent or above. It was often noticed that 

 the soil of Tumamoc Hill is full of spores of fungi and bacteria, 

 which develop very rapidly as soon as sufficient moisture is present. 



For comparison, a number of seeds of cultivated plants were tested 

 in the same way. Mexican beans (Phaseolus) and wheat (Triticum 

 vidgare) germinated in 15 per cent and more vigorously in 20 per cent 

 soil. The cultivated balsam (Impatiens) germinated slightly in 20 per 

 cent but much better in 25 per cent. Radish (Raphanus sativus) failed 

 to germinate in drier soil than 20 per cent. Red clover (Trifolium 

 pratense) failed to germinate until a moisture content of 25 per cent 

 had been reached. Thus it appears that of these plants the bean and 

 wheat are able to germinate with as scanty water supply as can 

 Fouquieria. Balsam and radish require more water than these, and 

 clover still more. It is probable that the seeds of typically desert plants 

 possess no greater power to germinate in dry soil than many plants of 

 the humid regions. Adaptation to arid climate does not appear to be 

 well marked as far as germination is concerned. 



TRANSPIRATION OF DESERT PLANTS. 

 THE GENERAL PROBLEM. 



Whether transpiration is a directly necessary function in plants may 

 be regarded as an unsettled question. By some it is considered as 

 essential in the transport of dissolved salts from the roots where they 

 are absorbed to the upper growing regions, and also in the cooling of 

 green parts when exposed to bright sunlight. By others transpiration 

 is considered as only a necessary evil, an evil because it increases so 

 greatly the amount of water necessary for plant life, and necessary 

 because in order to absorb carbon dioxide from the air, wet membranes 

 must be exposed. This must allow evaporation and thus necessitate a 

 renewal of water to the absorbing surfaces within the leaves. Notwith- 

 standing the emphatic denial by Burgerstein (1904) that there is any 

 reason in the position of Reinitzer (1881), Oels (1902), and Haberlandt 

 (1892) , who have expressed themselves more or less definitely as favor- 

 ing the second of the hypotheses outlined above, the question must not 

 be regarded as settled without conclusive experimental evidence, which 

 Burgerstein is noticeably unable to adduce. So far it seems practically 

 impossible to check transpiration absolutely by inclosing the plant in 

 supposedly saturated air under bell jars and the like, on account of the 

 fact that the absorption of heat by the green leaves must usually raise 

 their temperature slightly above that of the surrounding air. There- 

 fore the only method of experimentation which is available for study- 

 ing this problem is that of increasing or decreasing transpiration and 



