AGRlCtTLTTJRAL BOTANY. 329 



up tlio study of llie pl.iiil. Till' iniiiii physical ractnr wliifli was found to 

 (icItTiiiiiK- till' nature of the vej^etalion oil 'ruiiiainoi' Hill near the laboratory 

 was the water relation. Except duriiii; tiie i-ainy seasons, the soil on thid 

 hill is far too dry for most i)lants. and only |]ios(> forms can live there that are 

 ada[)tetl to tlry soils and a hiiih I'ate of e\aporation. 



Studies of the mininunn water suiii)ly for fjorniination of seeds were also 

 made. With desert plants such as /'(UKjiiicria xplciidciis and the fihint i-aetu^M 

 germination took ])lace in soils eontaininj,' from 1.") to 12() ])er cent of moisture. 

 For purposes of comparison a number of seeds of cultivated ])lants were 

 investigated in the same way. and it was found that Mexican beans and wheat 

 germinated in l."> |)er cent, but more vigorously in 20 per cent moisture in the 

 soil; the cultivated balsam germinated slightly in 2(» ])cr cent. Imt nnicli better 

 in 25 per cent; radishes failed to germinate in soil containing less than 20 

 lier cent moisture, and red clover did not germinate until a moisture content 

 of at least 25 per cent had been reached. 



lu summarizing his conclusions, the author states that the deei)er soil 

 layers of Tumamoc Hill contain at the end of the dry season a water content 

 adequate to the needs of those desert plants which are active during the months 

 of drought. The soil moisture is conserved very largely by the high rate of 

 evaporation and the conseciuent formation of a dust mulch. 



Desert plants show an adaptation to existence in dry soil, being able to exist 

 in soils somewhat drier than those needed by ])lants of humid regions, but 

 this adaptation is comparatively slight and is not considered of primary 

 importance. The downward penetration of precipitation water is slow in the 

 soil itself, but comparatively rai>id on the whole on account of the presence of 

 numerous obli(iue rock surfaces along which the flow is not markedly impeded. 

 By the nuddle of the summer rainy season all of the soil except the first few 

 centimeters is sufficiently moist to allow the germination and growth of most 

 plants. The seeds of FoiK/iticria si)lrii(leii-s and Cerriis f/if/onfnis failed to 

 show any special adaptation to germination in soils drier than those needed 

 by such seeds as beans and wheat. 



Immediately following germination the seedlings of desert plants exhibit 

 r. slow aerial growth, but an exceedingly rapid elongation of the ])rimary 

 roots, so that these should soon attain to depths where moistui'e is alway.s 

 ]»resent in an amount adequate for growth. The high moisture-retaining jtower 

 of the soil where these investigations were carried on holds near the surface 

 much of the water received from single showers and offers excellent oj)portunity 

 for the rapid absori)tion of this by shallow-growing plants, such as the cacti. 



In tlie conduct of these exix riments the author found that the effect of air 

 currents in increasing evaporation and transpiration is so great that measure- 

 ments of n.ntural transpiration can not be made in closed chambers, and by 

 me;ins of a si)ecially devised apparatus a physiological regulation of the rate 

 of transpiration was shown to exist in the forms studied. The regulation of 

 transpiration seems to be controlled by air temperature, the checking of water 

 Mjss beginning to be effective between 7'.)° and !KP F., and disaiipearing between 

 75° and S0° F. 



A study of the variations in the nitrogen and phosphoric acid in the 

 juices of succulent plants, <i. ANORf: (Conipt. Rend. Acad. Sci. [Paris i, lJi2 

 (lHOd). \<j. J'). PI). !l(Ki-!li)'i). — A study of the variation in these constituents in 

 McfKinhriauthonum (■ri.stallininn at different stages of growth showed that the 

 soluble phosphoric acid and nitrogen reach(>d their maxima at the same periods 

 i>\' growth. The content of nitric acid in tlu' juice was always comparatively 

 large. 



