48 Bulletin 78 



hot days than thermometers situated outside of their foilage would 

 indicate. 



The seed of most crops will germinate only during one or more 

 definite portions of the year while the temperature remains within 

 certain limits. For the seed of some crops this period is during 

 the cool part of the year and for the seed of others it is during the 

 warmer part of the year. Seeds of the former class either decay 

 or remain dormant through the portion of the year during which 

 the temperature is too high for germination and seeds of the latter 

 behave similarly during the cooler portion of the year. The seed of 

 a very few crops germinates here promptly during all parts of the 

 year if supplied with water and of a few others the seed germinates 

 during all of the year, except the hottest weather of summer and the 

 coldest weather of winter. 



Most crops make growth only during the portion of the year 

 that the temperature remains within certain limits, maturing, 

 dying, or becoming dormant when the temperature falls too low 

 or rises too high. Most annuals grow continuously during a certain 

 portion of the year and either die or mature w^hen the weather 

 becomes too cold or too warm, as the case may be. A few become 

 dormant as unfavorable weather comes, resuming and finishing 

 growth when the weather again becomes favorable. Most deciduous 

 perennials grow during one portion of the year only, while most 

 evergreen perennials make fresh growth during two distinct periods 

 of the year, remaining dcrm^ant or being killed back during other 

 portions of the year. 



General Efccts of Direct Sunshine 



Direct sunshine has an effect upon plants different from the 

 effect of diffuse sunlight at the same temperature. Any solid sub- 

 stance that intercepts the sun's rays becomes heated thereby to a 

 greater or less degree. A shaded object does not become as warm 

 as one exposed to the direct rays of the sun in an atmosphere of the 

 same temperature. This is due to the absorption of radiant heat 

 from the sun by the exposed object, as previously explained, while 

 the shaded object becomes heated only by contact with the warmer 

 atmosphere. In the shade, therefore, not only is the temperature of 

 the air lower, but absorption of heat from the direct rays of the 

 sun does not occur. Hence, the difference between the temperatures 

 of the soil and of objects in direct sunshine, and of the soil and of ob- 

 jects in shade is considerably greater than the difference between the 



