20 Wisconsin Academy of Sciences, Arts, and Letters. 



THE CAUSES AND THE OCCUEEENCE OF EXCENTBIC EADIAL GEOWTH. 



In a study of the distribution of excentrie radial growth, on 

 trees it is well to note that exeentricity may conceivably come 

 about in one or more of four ways and that in a sense such an 

 uneven growth of a stem at any height corresponds to the wave- 

 like uneven distribution at different heights of a tree. The four 

 ordinary ways excentrie stems may be built up are (1) by the 

 entire omission of radial growth in a part of the circumference, 



(2) by the unequal rate of growth on different sides of stems, 



(3) by the entire omission of summer growth on one side and, 



(4) by the omission of spring growth on a part of the cir- 

 cumference and its occurrence at other places. In looking over 

 papers on excentrie stems, etc., it is sometimes difficult to deter- 

 mine to which of the four classes the ease under consideration 

 belongs but usually that is apparent. 



Gravity and other factors of the environment as well as the 

 anatomic or physiologic characteristics of a species seem to be 

 the causes of excentrie radial growth but as yet the matter is 

 not fully understood. That a difference may be found in trees 

 of different groups in regard to excentrie growth, when subjected 

 to the same environment, is shown by some observations by 

 Nordlinger. 34 He cites an instance in which saplings of conifers, 

 beech, and oak had been bent over by the heavy snows of 1868 

 and afterwards grew in slanting positions. Three years later 

 sections taken at any point of the stems showed that pine, spruce, 

 and larch had developed three excentrie rings with the larger 

 radius below while on the oaks and beeches the three last rings 

 were thicker above. In one spruce only one very narrow ring 

 had been laid down on the upper side while the other rings had 

 been wholly omitted on that side. In both oak and beech radial 

 growth had been extremely slight on the under side during the 

 three years. This shows that different trees subjected to the 

 same environment may respond differently. That is, the specific 

 characteristics of a plant to a certain extent determine the man- 

 ner of response to the environment. 



Miiller's 35 observations seem to indicate that if excentrie 



" 4 I. c. 



"Miiller, N. J. C. Beitrage zur Entwicklungseschichte der Baum- 



■one. Bot. Untersuchungen 1:512-24. 1877. Heidelberg. 



