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 Nov. 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 
