214 Variations in Growth-Rates [412 



criteria,, weight and area, show variations that correspond 

 rather closely. Both of these show high rates for the sum- 

 mer and low ones for the spring and autumn. Judged by 

 dry weight of plant produced the growth-rate reached its 

 maximum (1.34 g. per plant, in 28 days) with the period 

 ending May 22, but this value remains high until after the 

 period ending July 31. Judged by the total leaf area, the 

 rate does not attain its maximum (214 sq. cm, per plant, in 

 28 days) until later, this occurring with the period ending 

 July 3, but this value is high for the three preceding periods 

 and for the two following. Roughly speaking, it may be said 

 that these two criteria give rates that are proportional, and 

 that they agree in indicating a period of very rapid growth, 

 extending from about May 8 to about July 17. Before the 

 period with its middle at May 8 the rates are lower, forming 

 a generally ascending series, from the very low values of the 

 early spring, and after the period with its middle at July 17 

 they decrease rapidly (with a low secondary maximum indi- 

 cated for the period ending Sept. 25) to very low values in 

 the autumn. 



The rates of growth in height fail to show this sort of 

 seasonal march; the maximum rate (49 cm. per plant, in 28 

 days) being shown for the period ending July 3, but this 

 rate also has very low values for the periods ending March 



13, Oct. 23 and JSTov. 6. By this criterion, the maximum for 

 the period considered (72.5 cm. per plant, in 28 days) occurs 

 with the period ending Sept. 25, but pronounced secondary 

 maxima are shown for the periods ending May 22 and Aug. 



14. This rate of growth in height appears to vary consider- 

 ably from period to period, but in a manner entirely inde- 

 pendent of the general advance of the season and quite inde- 

 pendent of the variations in rates of increase in dry weight 

 and in leaf area. As far as these data go, it therefore ap- 

 pears that there is nothing in the usually uncontrolled ex- 

 ternal conditions of a greenhouse in this climate, that may 

 be expected to produce a regular march of growth-rates in 



