Mat 11, 1906.] 



SCIENCE. 



749 



States Geological Survey, as recently ex- 

 pressed, appears to strongly favor the use of 

 alluvial slope, which thus takes its place in 

 the genetically related series, alluvial fan, 

 alluvial slope and alluvial plain. 



C. E. SlEBENTHAL. 



SPECIAL ARTICLES. 

 THE NORTHERN LIMIT OF THE PAPAW TREE. 



While the • flora of the upper Mississippi 

 Valley was yet in its primeval condition I 

 had good opportunities to observe the north- 

 ward geographical extension, and apparently 

 the northern limit, of certain plants which 

 reach full maturity of growth and fruitage 

 farther south. Although the floral conditions 

 which then existed in that region have been 

 in part modified by the progress of civilization, 

 the chief of the following statements are based 

 upon conditions which still exist. Among the 

 plants referred to is the papaw, Asimina 

 iriloha, which reaches its maximum size, that 

 of a small tree, and its most abundant fruit- 

 age in the broad region of which the mouth of 

 the Ohio River is near the center. Two of 

 the many other trees which are associated 

 with it there and which have accompanied it 

 in their northward dispersion, are selected for 

 special comparison. These are the persim- 

 mon, Diospyros Virginiana, and the pecan, 

 Cai-ya olivceformis. 



It is of relevant interest to note that al- 

 though these three species are commingled in 

 the same flora in the valley of the upper Mis- 

 sissippi, their post-glacial dispersion into that 

 region seems to have been from a pre-glacial 

 flora which occupied the papaw 'area before 

 mentioned and which was in part made up of 

 trees from different districts. The districts 

 which thus furnished the persimmon and 

 pecan respectively are assumed to have been 

 identical with their present respective areas 

 of gTeatest abundance and fruitfulness. That 

 is, the center of the area of greatest abundance 

 and fruitfulness of the persimmon may be 

 designated as within the state of Virginia, 

 which is far east of the similar center of the 

 papaw area, while it is in southern Texas and 

 the adjacent part of Mexico, far from both the 



persimmon and papaw centers, that the pecan 

 reaches its greatest abundance and perfection. 

 The persimmon apparently spread westward 

 into the papaw area and thence northward; 

 while the pecan ranged up into the Mississippi 

 Valley, traversing the papaw area, and thence 

 at least as far north as the forty-first parallel 

 of north latitude, a thousand miles from the 

 region of its fullest development. The un- 

 aided dispersion of the papaw seems to have 

 been proportionately less than that of either 

 the persimmon or pecan. 



In 1846, pecan trees of moderately large 

 size were yet growing and bearing fruit in 

 fair abundance ten miles above Burlington, 

 Iowa, and a nimiber of persimmon trees of 

 moderate size were also then growing and 

 bearing fruit in its season a few miles below 

 Burlington, on the Illinois side of the great 

 river. Neither of these two trees was then 

 common in that region and, so far as I could 

 ever learn, the localities mentioned constituted 

 the northern limit of their dispersion. It is 

 pertinent to my present purpose to mention 

 that both of those trees retained their fruiting 

 function unimpaired in their most northerly 

 extension, although the case was very different 

 with the papaw. During many years I ob- 

 served the last-named plant growing as a part 

 of the local flora at numerous localities along 

 the banks of the Mississippi, from northeast- 

 ern Missouri to the forementioned locality 

 north of Burlington, where the pecan grew, 

 the distance between the two extreme localities 

 being about seventy-five miles. In the south- 

 ernmost of the Missouri localities referred to 

 the plant reached almost arborescent size and 

 frequently, but never abundantly, bore and 

 matured its fruit. From there northward, 

 however, although the vegetative growth of 

 the plants was apparently healthful, they never 

 fruited, and gradually diminished in size to 

 shrubs, a few feet in height. It is true that 

 some thrifty specimens which grew upon the 

 Iowa bank of the Mississippi River, a few 

 miles above Keokuk, occasionally flowered, 

 but, although I frequently examined them in 

 the flowering and fruiting seasons, I could 

 never find any evidence that fruit had been 

 formed, or that any ovaries had ever become 



