Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 287 



it occurs about the larger bodies of water, and the gradual develop- 

 ment of succeeding meadow associations follows closely in the wake 

 of the bulrush-reed-grass combination. But with the smaller lakes 

 and ponds which are subject to rather wide fluctuations in water 

 level, a different vegetative condition is frequently revealed. The 

 portions of the lake or pond bottom exposed as the water recedes 

 are often covered by a dense mass of algae and aquatic mosses and 

 spermatophytes which linger for a time in the rich mucky substra- 

 tum. Such areas often retain certain aquatic species that are able 

 to adopt an amphibious life and these species frequently become con- 

 trolling and result in the production of a vegetative type which is 

 intermediate between the marsh proper and the wet weadow. Very 

 frequently the initiation of this sort of control and of this associa- 

 tion is made possible by the presence of Polygonum antphibium or 

 P. hartwrightii as members of the pondweed association or as sec- 

 ondary species of the bulrush-reed-grass association. As the water- 

 table recedes these species (or this species) persist while practically 

 all the aquatic associates perish and the result is the production of 

 a belt composed almost exclusively of the Polygonum. 



The development of the hydro-mesophytic mode of life by these 

 species is a fact of every-day observation. In the marshy areas of 

 the sandhills one can trace a perfect series of changes from the 

 typical form called P. amphibium through P. hartzvrightii and 

 finally to P. emersum. The first two forms, though often very dif- 

 ferent appearing plants, may commonly be collected from the same 

 rhizome. The latter "species" is almost as variable and it seems a 

 plain case that these three "species" are merely extreme variations 

 that may be found arising from the same rootstock. The possibility 

 of variation and successful reaction to the above great changes have 

 provided this plant with the means of quickly unfolding a complete 

 vegetative cover in these places. It often happens that P. hart- 

 wrightii or P. amphibium is to be found in great abundance over 

 the lower portions of the old lake bed while a trifle higher, in a soil 

 a little more dry and sandy, occurs a pronounced belt of P. emersum. 

 Sometimes the three forms are developed in such confused variation 

 and distribution that it is quite impossible clearly to differentiate 

 the three as to substratum requirements. Both P. hartwrightii and 

 P. emersum are common members of the next formation but neither 

 is developed there to the degree that marks the above situations. 



The individuals of this association are so closely grouped that 



