BUDS AND BBANCHES 95 



one branch out of ten had any live twigs. The sunlighted 

 branches, then, had 32 times as many twigs on the portions 

 counted as the shaded ones did. A similar study of a large 

 thorn bush {Crcdcegus) gave for the ten sunlighted branches 

 74 live twigs and for the shaded ones only 2, or 37 times 

 as many for the sunny side. 1 A study of the relative amount 

 of growth of the tips of branches during the year preceding 

 the observations showed that those on the sunny side of the 

 thorn grew 21 times as fast as those on the shady side. 2 



86. Definite and indefinite annual growth. In such trees as 

 the hickories, walnuts, butternuts, elms, poplars, and so on 

 (Figs. 72, 74, 82), the branches usually produce vigorous, 

 well-matured buds at their tips ; that is, they form definite 

 shoots, and each terminal bud develops promptly in the spring. 

 But some trees, like the honey locust, and such shrubs as 

 sumachs, roses, raspberries, and blackberries, form indefinite 

 shoots, which grow until their tips are killed by the frost. 

 Trees of this sort necessarily have a top much broken up into 

 minor branches. Why? 



87. Fruit spurs. A fruit spur is a short fruit-bearing twig 

 borne on the side of a branch (Figs. 76 and 77). Apple, pear, 

 plum, and cherry trees afford capital examples of the production 

 of fruit spurs. At the tip of the spur a flower bud (or a mixed 

 bud) is borne, and this usually develops into a cluster of flowers, 

 one or more of which may mature into fruit. In the apple and 

 pear (Fig. 77), though the flower bud contains a good many 

 blossoms, only one fruit is generally produced from each bud. 

 In cherries a single bud produces a cluster of fruits. Why ? 



If the terminal bud of the spur contained leaves as well as 

 flowers, a leaf bud is likely to grow in the axil of one of the 

 leaves and thus provide for the growth of the spur during 

 another year. This process may go on for a good many years. 



1 Comparing three-year-old portions of branches. 



2 The illumination in the shade (measured by "solio" photographic 

 printing paper) was, for the box elder, about one twentieth and for the thorn 

 about one eightieth that of the sunny side at noon in early July. 



