10 THE PLANT WORLD 



The two kinds of branches, as shown in the plate, are quite distinct, 

 and there seems to be no intermediate stage. One is characterized by a verj' 

 slow growth, seldom more than one-eighth of an inch in a year ; this ex- 

 treme shortening causes the really three-ranked leaves to appear fascicled. 

 The leaf scars of succeeding years are also brought very close together, 

 thus giving the shoot its rough, corrugated appearance. These rough 

 shoots are seldom more than one inch long, and are more commonly 

 seen on the large branches in the inside of the tree. (Plate 3, fig. 3.) 

 Branches of this description can be found which, judging from the number 

 of leaf scars, are at least ten years old, but which during that time have 

 made a growth of less than one inch. 



The other style of shoot, on the contrarj^ grows rapidly ; the leaves 

 are plainly three-ranked, often one and a half or two inches apart, with 

 smooth bark between them. (Plate 3, fig. 2.) 



It would appear that there must be some physiological reason for this 

 pronounced dimorphism, and it seems reasonable to look upon the amount 

 of light a shoot receives as the determining factor. The short shoots may 

 be considered somewhat as adventitious or dormant branches, which, 

 without encumbering the tree with useless material, and without expend- 

 ing energy on leaves that elaborate little nourishment, are in readiness to 

 produce a branch whenever the conditions change and a branch in that 

 particular place becomes desirable. This would be, of course, the case if 

 light were present. 



These dormant branches, as they may be called, are also to a certain 

 extent self -pruning ; if after a number of years no branch seems to be 

 called for, that is, if there is not sufficient light, the shoot becomes dis- 

 couraged and drops. 



If, on the other hand, a shoot that is receiving plenty of light and 

 consequently making rapid growth is thrown in the shade, the process is 

 reversed and the shoot at once ceases its rapid growth and goes into a 

 dormant state awaiting a return of the stimulating light. If this should 

 occur the branch will again resume its normal rate of growth. (See plate 3, 



fig. 1.) 



An apparent exception to the exposed shoots making rapid growth 

 may be noticed in what appear like fascicles at the end of many of the 

 rapidly growing branches. When the leaves appear in the spring they 

 stand close together and are separated afterward by the elongation of the 

 shoot. This seems in most cases to take place more rapidly at the base 

 of the year's growth, so that frequently a few leaves at the tip of the shoot 

 fail to become separated. It was not observed whether the fascicled leaves 

 of the dormant branches were entirely due to the failure of the shoot to 

 elongate during the summer or whether the leaves from the first are borne 

 in a different manner. The greater number of leaves borne on the rapidly 



