PLUMS IN" RELATION TO PRUNING 11 
tendency to form spurs is stronger with the Japanese group, and the 
production of spurs at fruiting nodes is more common. 
The number of years during which spurs live and continue to fruit 
varies greatly with the variety and with the environment. In gen- 
eral, the Japanese varieties retain spurs longer than the domestica; 
but within each of these groups, and more especially in the domestica, 
there is a wide difference among the varieties as to the performance 
of the spurs. If not weakened by shade or excessive crops, spurs of 
most Japanese varieties remain thrifty and productive for several 
years. The Burbank and Formosa are examples of Japanese varie- 
ties which retain their spurs well down along the old wood (see pi. 
7, B, and pi. 9, D). With the former this is due to the fact that the 
tree is naturally spreading and therefore admits light to the interior 
of the tree, unless pruning to induce a compact top is practiced. The 
trees of the latter, although naturally upright, branch but little and 
tend to remain open. Other Japanese varieties, including the Santa 
Rosa and Beauty, make a compact, erect top growth if the trees have 
been headed back in pruning. The shade in the interior of the tree 
results in slender spurs, many of which are short lived. If these trees 
are kept open to admit light the spurs are retained well. Trees of 
some other varieties, of which Duarte and Eldorado are examples, 
send out slender branches freely at a wide angle to the main branch 
during their first or second season's growth. These slender branches 
produce many spurs and retain them well, as they withstand more 
shade than those on most other trees where the new growth is 
naturally upright. 
Of the domestica group, the varieties which are distinctly spur 
forming, such as the Jefferson, Sergeant, Imperial Epineuse, Presi- 
dent, and California Blue, retain their spurs over a long period (see 
pi. 17, D ; pi. 26, A ; and pi. 31, A). The spurs are prolific and with- 
stand shade rather well. Because of the naturally open growth of 
spur-producing trees they are usually well supplied with light. Va- 
rieties which produce twigs rather than spurs often lose any spurs 
which are produced after a very few years, at most, if the trees are 
making a thrifty growth. They soon grow to twigs if not fruiting, 
and fruiting spurs often die after the fruit harvest if they do not 
grow to twigs the same season (see pi. 25, C). Many trees of the 
twig- forming type produce fruit heavily on 1-year-old spurs. Very 
few of these fruiting spurs produce leaves, and they die after the 
fruit ripens. Spurs on such trees are easily injured by shade, and 
many of them are lost from this cause, as the growth of twigs and 
branches usually results in a compact tree if a thrifty wood growth is 
maintained. With some varieties, notably Golden Drop and Presi- 
dent, the tendency is for the spurs to be short lived if the annual 
growth is poor. On such trees the 1-year-old twigs and spurs are 
very prolific, and after the fruit ripens many of the 1-year shoots 
are left barren and many 1-year spurs lost. If the trees grow vigor- 
ously the shoots are large and fruitfulness is somewhat delayed, 
permitting the formation of a greater number of spurs, which become 
well established on the new wood (see pi. 31, A). 
Unfavorable growing conditions are indicated if spurs form at 
the end of twigs of varieties which are inclined to produce twigs 
instead of spurs, and it is only by the improvement of growing con- 
