12 BULLETIN 1477, U. S. DEPARTMENT OP AGRICULTURE 
ditions of such varieties that regular and good crops of large-sized 
fruit from them may be expected. Drought and insufficient pruning 
are the usual causes for this type of growth, although poor soil 
or root or branch diseases (pi. 37, B) ma}^ give similar results. It is 
encouraged also by little or no cutting back of the new wood in 
priming and hastened by heavy fruit production. 
If trees normally spur bearing produce more twigs than are de- 
sired, the excessive growth may be corrected by substituting thinning 
of branches for cutting them back in pruning. If spurs are inclined 
to grow to twigs readily, as they do on the Tragedy, Imperial 
Epineuse, and Sergeant under favorable growing conditions, less 
cutting back and more thinning out to admit light is needed than 
where the new growth is normally short, as is the case with the 
Jefferson. 
Where spurs are lost from excessive shading, some time will be 
required in corrective pruning to establish new ones, since as a rule 
it is desirable to establish new shoots and twigs on which spurs are 
to develop. If the trees have been kept low, the outer shoots may 
serve for the fruiting wood and be induced to develop spurs if enough 
of them are eliminated at pruning time to admit the necessary 
light; but for taller trees, heading back and removing enough old 
branches to induce new shoots to spring from the old wood are 
necessary. In such cases it is well to establish fruiting wood nearer 
the ground. 
Some varieties retain the vegetative vigor of the old wood 
much better than others, and such varieties are most inclined to 
develop new spurs on old wood. With these, less heading back 
is necessary to induce new growth. In general, the Japanese vari- 
eties replace spurs which have been broken off or those lost by 
natural causes much better than the domestica, although there is a 
decided variation among Japanese varieties in this respect. Those 
which produce short fruiting twigs most freely by the outgrowth of 
spurs also renew lost spurs on old wood more readily when favor- 
able conditions for spur development recur. Trees of varieties on 
which the main branches send out numerous long, slender twigs, along 
which most of the spurs of the trees are formed, do not replace lost 
spurs readily; and cutting back in pruning or other changes in 
treatment sufficient to cause the growth of new branches and twigs 
is needed. The Duarte, Eldorado, and Santa Eosa are the best 
examples of those which produce many long, slender twigs and on 
which spurs tend to remain small and do not grow to twigs. The 
Satsuma and Gaviota are perhaps the best examples of those whose 
twigs grow readily from spurs. 
In the domestica group the variation in regard to the growth of 
twigs from spurs is more pronounced than in the Japanese group; 
although, in the former, varieties most inclined to send out twig 
growth from the spurs are also most inclined to replace injured spurs 
(see pi. 17, C). The Imperial Epineuse, Sergeant, and California 
Blue illustrate this. With other typically spur-bearing varieties 
where little, if any, twig growth appears on the spur unless some 
living wood has been cut back or removed in pruning, spurs broken 
off are seldom replaced (see pi. 23, D, and pi. 29, C). Jefferson 
