142 ILLINOIS ACADEMY OF SCIENCE. 



do not explain the establishment of the species. For this estab- 

 lishment we must find the seedlings, and none exist upon the 

 moving dune complex. 



REPRODUCTION BY SEED. 



In the experience of the writer, cottonwood seedlings are to be 

 found in the dune region in two situations, and in two only. In 

 the recession of the waters of Lake Michigan, many shallow 

 ponds and lagoons were cut off from the main body of water 

 and not a few still persist, more or less filled by the action of 

 the moving sand and vegetation. Their damp margins are often 

 sprinkled with cottonwood seedlings (Fig. 4) ; some of these 

 survive and surmount the advancing sand and thus account for 

 the presence of these trees at any considerable distance from 

 the lake. 



The second seed-bed is to be found where the wind has swept 

 out the sand to a depth approaching the level of the waters of 

 the lake; here the soil surface and the top of the water table 

 almost coincide in depressions termed by European ecologists 

 pannes. Such pannes are (Fig. 5) often found immediately 

 behind the fore-dune within 25 to 100 meters of the shore. These 

 depressions are flooded at high water in the spring and even at 

 midsummer the almost saturated sand is quite stable and seems 

 admirably suited to the production of cottonwood seedlings, for 

 here they are found in abundance. As they increase in size they 

 collect sand, the panne being transformed into an ever-increasing 

 dune (Fig. 6), which soon begins to move inland. With the 

 growth of the dune and its subsequent advance many of the 

 young trees are killed, but some survive and, surmounting the 

 advancing sand, form the only tree vegetation of the active dune 

 complex. 



CONCLUSIONS. 



1. The evaporating power of the air in the exposed cotton- 

 wood dune association is great in amount and variable in degree, 

 indicating rigorous atmospheric conditions. 



2. The soil moisture is never less than twice the wilting coeffi- 

 cient of the soil ; i. e., there is always a supply of water available 

 for plant growth. 



3. Vegetative reproduction maintains the stand of cotton- 

 woods upon the dunes through the ability of the species to send 



