Growth of Tree Twigs 129 
it had been practical to take one or two earlier sets of measurements. 
The type of curve is shown best of all in the unshaded shoots of peach 
tree No. 1. 
It will be noted that peach tree No. 1 and group 1 of the dogwoods 
reached their maximum growth rate during the week ending on May 
31st. These trees were under very similar conditions. However, peach 
tree No. 2 grew much more rapidly early in the season but ceased grow- 
ing sooner. The shaded dogwood reached its maximum earlier than the 
other but because of an earlier decline in growth and not because of 
a more rapid early growth. 
The behavior of these two peach trees seems to point toward the 
conclusion that the early spring rate of growth is influenced a great 
deal by the temperature. Tree No. 2 had a considerably warmer loca- 
tion than No. 1. It should also be remarked in this connection that 
there was freezing weather on one or two occasions after these meas- 
urements were begun. It may be possible that if optimum temperature 
conditions had prevailed early in the season the growth rate would 
quickly have approximated that found later, after making due correc- 
tion for the number of working hours per day which the plant has at 
the two periods. 
The cause of the onset and continuation of a decreased growth 
rate was clearly not lowered temperature, however, because growth had 
stopped while the temperature was still at a point at which rapid 
growth occurred earlier in the season. The evidence from the rain- 
fall is that variation in the water supply was at least partially re- 
sponsible. It seems reasonable to suppose that in the case of peach 
tree No. 1 and dogwood group No. 1 the growth rate would have con- 
tinued to augment for some time longer but for the onset of drier 
weather. In the case of the shaded dogwood the decrease in growth 
was probably caused by shading. Although no record was made of 
the time when the forest trees had expanded their leaves sufficiently 
to produce full shade it was noted that growth in the shaded dog- 
wood seemed to stop shortly after this occurred. It was also noted on 
the peach trees that as soon as a branch became shaded it quickly 
ceased growing. This was true whether the shoot came from a last 
season’s bud or was a lateral on a shoot of this season’s growth. This 
observation would account, at least partially, for the more rapid de- 
crease in the growth rate of branches which became overtopped, as 
shown by the curves for the two peach trees. 
In view of the work of Garner and Allard’ on the effect of the 
relative length of day and night on the onset of the flowering stage of 
plants it is worthy of suggestion that this factor also may be operative 
in such cases as this. It is a common observation that hardy woody 
plants usually cease growing and harden their twigs while the weather 
is still warm enough for good growth, whereas in the spring they 
begin growing very shortly after the weather warms up sufficiently. 
2Garner, W. W., and H. A. Allard. Effect of the relative length of day and night 
and other factors of the environment on growth and reproduction in plants. Jour. 
Agr. Res. 17:553-606. 1920. 
9-20320 
