Growth in Eriogonum nudum 



219 



The soil temperatures of this situation were very even, ranging during 

 the summer at 18 cm. depth between 21 degrees and 23 degrees C. The 

 maximum is reached late in the afternoon, not earlier than 20 hour, and 

 is due to transmission of heat from the upper to the deeper layers. The 

 18 cm. layer lags about 4-5 hours behind the 13 cm. layer. 



The soil is sand with a very slight admixture of humus; is, in fact 

 that of an old sand dune. 



Daily periodicity of growth. — A generalized conception of the daily 

 growth cycle of Eriogonum nudum under the conditions described is 

 conveyed by the graphs which exhibit the average hour rates for several 

 internodes for their entire growth periods (fig. 4) and for the whole 

 period of observation, 53 days (fig. 3). 



Hour g 



Figure 5. Hour rates of evaporation: 5, sunlight; /, high fog. 



Regarding particularly the latter it is seen that during the night 

 the average growth rate is somewhat less than .2 mm. Beginning with 

 the 4 hour there is a slight increase of rate till 6 or 7 hour after which 

 there is a slight decrease in rate. After 8 hour there is a rapid drop in 

 rate to zero, or practically so. At 10 hour there occurs an equally 

 rapid rise in rate, culminating at 16-17 hour when the rate attained 

 reaches a value approximately four times the night rate. After the 

 17 hour the rate falls again till the night value is reached at about 

 21 hour (Fig. 3). 



The critical features of the cycle arc (i) the amount and constancy 

 of the night values; (2) a usual but not invariable increase in rates after 



