Oct 29, 1921 Relation of Soil Temperature to Onion Smut 243 



in the Department of Plant Pathology, University of Wisconsin, for the 

 control of soil temperatures has been described by Jones (5) . Briefly, it 

 consists of a series of water baths held at constant or nearly constant 

 temperatures in which the glass or galvanized-iron culture pots are 

 inserted.! s ^^-f?-' .'i.suuu-.. 



For these experiments galvanized-iron cylindrical pots 5 inches in di- 

 ameter and 8 inches in depth were used. In order to overcome the in- 

 fluence of the air temperature upon the upper layer of soil, the surface 

 of the latter was kept at >2 to i inch below the level of the water. Tar- 

 paper covers were placed over the pots until the seedlings came above 

 ground; these covers were then removed, and a layer of mineral wool 

 was placed on the surface of the soil. By this procedure the tempera- 

 ture of the upper inch of soil was kept reasonably close to that of the 

 deeper portions — that is, approximately that of the water in the tank. 

 In order to follow any minor variations, readings were taken three times 

 daily from thermometers inserted i inch below the surface of the soil. 

 At the beginning of the experiments the moisture content of the soil was 

 adjusted to two- thirds of the water-holding capacity. The pots were 

 thereafter weighed at intervals of one to three days, depending upon the 

 rate of water loss, and the moisture content was readjusted accordingly, 

 either by adding water directly to the surface or by introducing it at the 

 bottom of the pot through a glass tube. Obviously this method did not 

 secure uniform distribution of moisture throughout the pot, and una- 

 voidably the content of the upper layer of soil was somewhat lower than 

 the average for the whole pot. It is believed that this variation had little 

 if any influence, however, since other experiments, described earlier in 

 this paper, showed that infection is quite uniform over a much wider 

 range of soil moisture than here occurred. The seed was planted at a 

 depth of I inch. Since the chlorophyll in the tops in some cases ob- 

 scured the smut lesions, the plants were placed in alcohol acidified with 

 acetic acid until thoroughly bleached before final examination for the 

 disease was made. 



EFFECT OF SOIIv TEMPERATURE UPON THE DEVEI.OPMENT OF THE HOST 



Experimental studies to determine the relation of soil temperature to 

 the rate and character of seed germination and seedling development 

 were carried on in conjunction with those relating to infection, of which 

 the results will be presented in the next section. It will be simpler, 

 however, to discuss these two aspects of the problem separately, taking 

 up first the relations of temperature to host development. 



Experiment I. — Seven pots of sterilized greenhouse loam soil were 

 uniformly planted with 50 Red Globe onion seeds in each pot. One pot 

 was then held at each of the following temperatures: 10° to 14°, 16.5° 



1 since this description was published, numerous improvements have been made from time to time by 

 members of the Department. As now in use these are termed the "Wisconsin soil temperature tanks." 



