OPLIIMS m RELATION - TO PRUNING- 13 
and Golden Drop are examples of this type. Such open trees usu- 
ally retain spurs well, however, and seldom lack for fruiting wood. 
Golden Drop is to a certain extent an exception to the latter rule. 
The branches of domestica varieties which are decidedly twig- 
bearing soon become barren by the dying out of twigs if conditions 
are unfavorable for wood growth (see pi. 19, C). In such places 
heading back to induce the growth of new shoots from which twigs 
may grow is necessary, as fruiting twigs do not grow freely from 
the older portion of the branch. 
EFFECT OF LOW SUMMER TEMPERATURES ON WOOD GROWTH 
In the group of Japanese plums and their derivatives a decided 
difference in thrift and habits of growth has been observed with 
different varieties growing in the cool, coastal region of California. 
With certain varieties, differences in thrift are noticed also between 
trees growing in the cool region and those of the same variety grow- 
ing in the interior valleys of California where the summer tempera- 
tures are high. Trees which have had their new growth severely 
and regularly headed back in winter and are growing in fertile 
irrigated soil have been used for comparison. 
In the region near Mies, Calif., the cool breezes from the San 
Francisco Bay maintain a temperature several degrees below that 
near Auburn or Vacaville, two of the important plum-growing sec- 
tions in the interior valleys of California. (Table 1.) In the 
cooler region the vegetative vigor of a few varieties is decidedly 
lower than that of others. With these varieties the new shoots 
growing from the previous year's shoots which have been headed 
back are slender and short and often reach but a few inches in 
length even in the tops of the trees; the spurs make but little ter- 
minal growth; new twigs which appear are slender; and spurs and 
twigs which are broken off or die are seldom replaced. Varieties 
which illustrate these weaknesses include Occident, Kelsey, Chalco, 
Gaviota, and Wickson (pi. 2, B and C). There is considerable 
variation, however, among these varieties, their comparative weak- 
ness being in the order named. These varieties contrast with May- 
nard, Santa Rosa, Burbank, Satsuma, and Apple, which make a 
very thrifty growth (pi. 3, A). Occident, Kelsey, and Chalco are 
inclined to lose their spurs and twigs early and the portion of the 
branch which is but a few years old becomes barren and remains so. 
The spurs of Gaviota also are inclined to die out early, but the twigs 
are moderately persistent and fairly thrifty. The spurs of the 
Wickson live and remain productive along the entire branch of even 
the framework branches, although they make but little lateral 
growth. Twigs and new shoots such as grow from branches that 
have been headed back are very short. Vigorous shoots have been 
observed growing from framework branches of trees of this variety, 
but the vigor of the smaller fruiting branches is low. 
In the interior valleys, where the maximum and mean tempera- 
tures during the summer months are decidedly higher than in the 
coastal region, all varieties of the Japanese group which have been 
observed grow well. 
