400 IOWA ACADEMY OF SCIENCE 



The distribution of these trees and shrubs is very interesting 

 and calls for the following description : 



Extending south from the Big Sioux to the foot of the bluff 

 is a tract of low river bottom subject to overflow, in width about 

 350 feet. At the south edge of this flat, the bluff rises abruptly, 

 reaching its crest 1,250 feet south of the river, at which point 

 it has an elevation of about 180 feet. All the soft maple trees 

 (Plate XII, figure I) are located on the river flat, and none of 

 them appear on any part of the bluff. This is to be accounted 

 for because the soft maple is a water lover, and finds an abun- 

 dance of moisture, and good protection from the dry southwest 

 winds of summer, under the shelter of the bluff. These trees 

 run to large dimensions, some of them being as much as twenty- 

 four inches in diameter. The American elms (Plate XII, fig- 

 ure II), 440 specimens, are divided into four principal groups. 

 The greater part are to be found in the thick timber on the 

 upper third of the bluff face. Below this, there is an interval 

 of several rods succeeded by a considerable interval thickly 

 timbered with the American elm. Then comes another interval 

 without elms, after which they are scattered rather freely down 

 to the foot of the bluff. The fourth group consists of a few 

 scattered individuals located on the river bottom, close to the 

 foot of the bluff, and made up of trees probably seeded from 

 the trees on the bluff face. There is a very conspicuous absence 

 of these trees from the greater portion of the river bottom, 

 which is to be accounted for by the river floods, by the sand 

 and gravel soil, and by the over supply of ground water. The 

 elm's ability to vary its transpiration and to withstand severe 

 evaporating tendencies in environment will be brought out in 

 the records of transpiration and evaporation to appear in an- 

 other paper. 



The black willows (Plate XII, figure III) are one hundred 

 ten in number, all grouped on the low ground near the river, 

 with the exception of a few trees about 100 feet from the river 

 bank. This grouping is readily accounted for by the tree's 

 fondness for water. These trees occur chiefly in clumps sur- 

 rounding a center where the parent of the group formerly stood. 



There are five box elder trees (Plate XII, figure IV), in the 

 tract studied, four of which occur on the edge of the river, and 

 the fifth one some seventy-five feet south of it, but all of them 

 so located on the river bottom that they are certain of an un- 





