372 The Living Plant 



control. From such elaborate structures as the Witches Brooms 

 there are all grades downward to those so inconspicuous as hardly 

 to attract notice, including some very simple kinds of excrescences. 



Results very similar to these may be caused by external me- 

 chanical injuries. Thus, when tree trunks are injured in the cam- 

 bium, this also loses its regularity of growth, and becomes thrown 

 into various contortions, producing gnarly fibrous growths which 

 often appear as large burls on the trunks. Moreover, the injured 

 cambium at times attempts to put out adventitious buds, which, 

 forming in large numbers but without proper control, just about 

 keep pace with the expansion oi the trunk. The resultant masses 

 of wood show the characteristic rings of buried branches, often in 

 patterns displaying great beauty, of which the Birdseye Maple 

 affords a conspicuous example. 



Disturbance of growth by Internal Causes. — In addition to the 

 external and visible causes which throw growth into confusion, 

 there are others which appear to be internal. One of the simplest 

 cases is that in which the correlation between the different parts 

 of a plant becomes weakened or lost. This correlation is beauti- 

 fully shown in the familiar fact that if the young tip of the main 

 stem of a Spruce or a Fir tree be cut away, one of the nearest 

 branches will grow up and take its place, although, had the tip 

 remained, that branch would have grown like its neighbors 

 horizontally outwards. It is indeed this correlation of the geot- 

 ropism of the branches which makes possible the symmetrically 

 radiate shape of a tree, each branch as it grows assuming the 

 correct geotropic angle to form either the cone or the ball of 

 foliage as the case may be. Now in old trees this correlation is 

 sometimes lost, perhaps through the interruption of the pro- 

 toplasmic connections along the stem, and then each new branch 

 grows upward precisely as if it were the only main stem, as our 

 accompanying figure illustrates (figure 143). Obviously such 

 cases are related in method with the Witches Brooms and the 

 like, earlier mentioned. Somewhat the same thing occurs in 



