115 



gravel, with :i few thin seams of light clay at various levels. Throughout 

 the area from which the oalis were cut, the soil overlying the drift ma- 

 terial ranges from eight to twentj^-three inches in thickness. So far as the 

 physical and chemical composition of the soil is concerned we have prac- 

 tically identical conditions over the entire area. 



The black walnut was cut for the most part in an area lying in the 

 second river terrace, where, in addition to the forest humus, there occurs 

 from three to five feet of alluvial soil before the clay is struck. The clay 

 also in this area is perhaps twice as thick as in the former case. The ter- 

 race has an eastern exposure, while the curves of the river protect the par- 

 ticular tract in question from the north winds, but leave it open to winds 

 from the south. Upon the west it is protected by the escarpment of the 

 upper terrace. The area covers but a few acres and evidently furnishes 

 as uniform conditions as can be found in nature. The two tracts present, 

 however, fairly distinct conditions, a fact which should be borne in mind in 

 any comparison of the rate of growth. 



The measurements of the diiferent species are given in tabular form as 

 furnishing in the main the data for the deductions drawn later in this 

 paper. Possible occasional errors may occur in the computation of per- 

 centages in spite of the fact that the figures have been reviewed three 

 times. 



From the tables it is shown that in the area indicated and under the 

 conditions outlined the average yearly increase in white oak, based upon 

 sixty specimens, was .1995 of an inch ; of red oak, based upon forty-nine 

 specimens, was .22674 of an inch ; of black walnut, based upon thirty-two 

 specimens, was .27712 of an inch. 



A number of interesting inferences seem plain. 



1. There is a wide range in the growth rate in trees of the same 

 species, even when growing under the same conditions. Thus the range in 

 w^hite oak is from .095 to .328 of an inch ; in red oak, from .134 to .515 of 

 an inch ; in black walnut, from .19.5 to .358 of an inch. Such wide range 

 under conditions so nearly identical must be referred to individual idiosyn- 

 cracies, probably referable in most cases to the vigor of the acorn, to the 

 character of the tree from which the acorn was derived, to inherited growth 

 tendencies or similar causes. An examination of the table of trees of simi- 

 lar age in respect to their diameters will show clearly this "personal equa- 

 tion" of the tree. For example, in Table II, numbers 38 to 45, inclusive. 



