583 



with Yapp, that plants may grow in proximity to each other and yet, 

 if vegetating in different strata above the soil surface, be subject to 

 widely different growth conditions. Thus, for e>tample, Riccia na- 

 tans and Typha latifolia, which may be found together in great quan- 

 tity but vegetate mostly in dift'erent atmospheric strata, live under 

 evaporation conditions dift'ering much more than do those under 

 which Tcucriuiii occidcntale (of the reed swamp) and Aster salici- 

 folius (of the swamp white oak -white ash forest), plants of similar 

 height and growth form, live. 



The depth of the water-table in the reed swamp and the swamp 

 meadow was observed each week from May 21 to October 22, 191 1. 

 The Avater in Skokie Stream was about i m. deep in May, after 

 which it gradually declined until in July, when the stream bed was 

 in most places fairly dry. In August the water began to rise again, 

 and by October had reached an average depth of about i.i m. In 

 the rest of the reed swamp and in the swamp meadow the water- 

 table during May was coincident with or above the soil surface; 

 thereafter it sank, until in early September the maximum depth of 

 I m. in the reed swamp and 1.75 m. in the swamp meadow was 

 reached ; and then, rising rapidly, it reached the surface again by the 

 middle of October. During 191 2, water was much more abundant 

 throughout the marsh. Seldom could the reed-swamp be traversed 

 without the use of boots, even in midsummer. According to farmers 

 in the vicinity of Glencoe, Skokie Stream has sometimes in the past 

 risen until a depth of about 3 m. was reached, when the entire marsh 

 was of course deeplv submerged. Various attempts have been made 

 to classify the constituent species of a formation with relation to the 

 optimum water-table depth for each species. But where the water- 

 table varies greatly in depth from month to month and from year 

 to year, data must be secured through many years if they are to 

 show more than merely the relative degrees of hydrophytism to which 

 plants in different places are subject. 



Litmus tests each week, from May 21 to October 22, 191 1, showed 

 the water in Skokie Stream to be either neutral or slightly alkaline. 

 Similar tests showed the soil water in the outer parts of the reed 

 swamp and in the swamp meadow to be usually neutral or slightly al- 

 kaline, except that for a few days in August acid was present, al- 

 though the amount was almost negligible. 



Subterranean Organs and their InterrEeationships 



A study of the subterranean organs of the reed swamp plants 

 showed that in many cases their depth is roughly proportionate to 



