THE DUNES OF LAKE MICHIGAN 215 



view and I wish to conclude my paper with this remark. We 

 have already treated of the similarity between xerophytic and 

 halophytic adaptations. Now we have seen, that certain swamp- 

 plants are able to thrive on a dune-soil, which is dry to such an 

 extent, that even most of the xerophytic dune-species are not 

 able to stand these conditions. No wonder then, that in many 

 books on oecology the supposition is admitted that swamp-plants 

 and xerophytic species also belong to the same class of adapta- 

 tions. 2 A priori it does not seem difficult at all to argue this 

 assumption. Swamp-soil, we read, is physically wet, of course, 

 but physiologically speaking it is said to be very dry, mainly be- 

 cause of the large amount of humous acids present in it. On 

 swampy soil the plants would therefore behave as if they were 

 living in xerophytic life-conditions. In other words, it is ex- 

 pected a priori, that swamp-plants and xerophytes are similar 

 adaptations and will be able to grow in each other's places. In 

 Holland there are certainly plenty of species, that seem to prove 

 the truth of this conclusion. Salix repens has already been 

 named and other examples can easily be cited. There is e.g., 

 Polygonum amphibium, which usually grows in ditches and 

 swamps, but often also on the driest places in the dunes. In- 

 versely several of our heath-plants, e.g., Calluna vulgaris and Em- 

 petrum nigrum, are to be met with in our peat-bogs. Very in- 

 structive in this regard are likewise the so called pine-bottoms, 

 which we studied at Dune Park, Indiana. When a coniferous 

 forest develops on established dunes, it usually happens on 

 windward slopes and summits. But in certain low swampy de- 

 pressions the pines may also appear and produce the pine-bot- 

 toms in question. Here the life-conditions are "oxylophytic," 

 there they are certainly xerophytic and again we could get the 

 conviction, that the two sorts of conditions are in reality iden- 

 tical. Still I am of the opinion, that we must be very careful in 

 assuming the correctness of this hypothesis. Indeed I do not 

 believe that swamp-soil may be called physiologically dry and I 

 should say, that the explanation of the remarkable fact, that so 



2 See e. g. E. Warming, Oecology of Plants, Oxford, 1909, p. 193 a. f . 



