CH. II DEVELOPMENT AND ENVIRONMENT 15 



automatically modifies the severitj?- of the environment, 

 in so far as the lower portions are concerned, by producing 

 a " surface climate" with higher humidities and less wind, 

 consequently less evaporation, and affording at least 

 intermittent shade to any given portion of plant tissue. 



Germination. — The sowing of the Egyptian crop takes 

 place at different times, varying with the locality and with 



Night 6 a.m. Noon 



I a. Seed aou/n b. Irrigated 



Weather normal 



6 p.m. 



Night 



Fig. 29. — Tempebatuee op the Seed Bed. 



Recorded by thermo-electric junction placed 5 cm. below surface on 

 south side of ridge. 



the weather, from the end of February until May. The 

 habit of sowing on ridges running east and west secures a 

 higher soil-temperature on the south face. The tempera- 

 ture of the seed bed before, during, and after optimum 

 sowing-time was recorded electrically in 1911 {Fig. 29); 

 the sudden rise shown at the moment of sowing (a) is due 

 to the scorched surface soil which fell into the hole, 

 showing how effective is the heat insulation of dry soil, 

 since the depth of the hole was only 4 cm. The heat 

 absorptivity and conductivity of the soil after irrigation (6) 



