119 



tain a diameter of eleven iuclies, or mal^e a net gain of eight inches. This 

 would give an average yearly increase of .4 of an inch, which is considerably 

 greater than the highest yearly increase in any of the sixty specimens ex- 

 amined, or .328 of an inch. jNIr. Fox also states that a three-inch red oak 

 sapling would, in twenty years, attain a "liameter of thirteen inches, thus 

 making a net gain of ten inches. This gives a yearly rate of .5 of an inch, 

 or more than double the average yearly rate (.22(374 of an inch) of the forty- 

 nine specimens examined. It is true that specimen 37 shows an average 

 yearly growth of .515 of an inch, but the next is .336 of an inch and only 

 six out of the forty-nine show an average yearly growth in excess of .3 

 of an inch. An examination of a number of white and red oak logs at 

 local mills confirms the conclusion that tlio growth rate in the area studied 

 is exceedingly slow. 



It is probable that the cause of this slow growth is to be found in 

 relatively thin soil underlaid by the hundred or more feet of drift. The 

 sand and gravel of the drift constitute a natural filter which rapidly car- 

 ries the soil water to lower levels. The thin soil and the stratum of clay 

 can not hold sufficient water to carry the trees through our long summer 

 drought and at the same time furnish a large amount of material for 

 growth. Observation of the trees of the Purdue campus furnishes confir- 

 mation of this view. The soil conditions of the campus are practically the 

 same as in the area studied. The older trees of the campus were set out 

 between 1875 and 1880, and were largely maples and elms along the drive- 

 ways, other forms being scattered through the gi'ounds. The maples and 

 elms are in sufficient numbers to justify a few generalizations. The trees 

 show an early period of rapid growth, a period of slow growth and finally a 

 practical cessation of growth. During this latter period the trees begin to 

 show all the signs of what might be called senility. In the early years, the 

 roots not having penetrated deeply, find sufficient available moisture in the 

 thin soil to provide for the maintenance of the tree and its normal growth. 

 A little later, the deeper penetrating roots reaching the drift find but little 

 water, so little, indeed, that under the most favorable conditions provision 

 can be made for only a slight growth. Still later the increasing demands 

 of the tree can not be satisfied and it begins to age, and we have the case 

 of elms and maples completing their life cycle in twenty-five or thirty years, 

 attaining in the meantime a diameter of from ten to fifteen inches. The 

 duration of life upon the campus is much less that in forest conditions, be- 



