736 



ECOLOGY 



and there is formed a dwarfed portion, the terminal bud, with closely 

 imbricated scales; in the autumn the leaves fall from the elongated 



portion, leaving the 

 widely separated scars. 

 Thus the age of a 

 branch may be deter- 

 mined by noting the 

 number either of the 

 dwarfed or of the elon- 

 gated regions. Some- 

 times the dwarfed 

 portions are thicker 

 than the elongated 

 portions. 



The possible factors. 

 The factors involved 

 in stem periodicity 

 probably are external, 

 but they have not 

 been determined ex- 

 perimentally. If the 

 terminal bud is re- 

 moved early in the 

 first season, the lat- 

 eral shoots elongate, 

 indicating that ordi- 

 narily the main shoot 

 for a time inhibits 

 elongation in the lat- 

 eral shoot, possibly 

 because it utilizes the 

 food necessary for the 

 development of the 

 latter (see p. 749). 

 However, when once 

 the lateral shoot be- 

 gins to develop, its 

 growth phenomena 

 appear to be con- 



1059 



FIGS. 1057-1059. Twigs, illustrating growth period- 

 icity and the characteristics of winter buds: 1057, a twig 

 of an ash (Fraxinus), showing alternating regions charac- 

 terized by slight stem elongation (viz. at b, the present 

 winter bud; at b', the position of the previous winter bud; 

 at b", the position of the winter bud of two years previous, 

 etc.) and by considerable stem elongation (viz. at a, repre- 

 senting the growth of the previous summer ; at a', represent- 

 ing the growth of the summer previous to that, etc.) ; note 

 the large size of the terminal bud (b) in proportion to that of 

 the lateral buds (c} ; /, leaf scar ; d, lenticels ; at c f are scars 

 left upon the fall of lateral buds of previous years ; 1058, a 

 twig of the cottonwood (Populus deltoides) ; note the large 

 buds with imbricated scale leaves (e); note also that the 

 lateral buds (c) are about as well developed as the terminal 

 bud (/) ; other lettering as in 1057 ; 1059, a twig of Catalpa ; 

 note the small winter buds, both terminal (/) and lateral 

 (c) ; note also the prominent circular leaf scars (/) with an 

 inner circle (v\ representing the position of the vascular 

 bundles severed upon leaf fall. 



