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189] DUNE FLORAS OF LAKE MICHIGAN 293 
in fig. 15, as at the left of the center, are Salix adenophylla. 
The lower shrubs are dogwoods or glaucous willows. Fig. 16 
shows a group of the latter two species growing together. How 
long these plants will be able to endure is a question that can- 
not now be answered. The conditions become severer each year, 
because of the necessity for increased stem elongation, and also 
because the plants are constantly rising above the protected 
position in the lee of the dune. At no place is the destructive 
action of the wind greater than at the summit of an advancing 
lee slope. : 
The encroachment of a dune upon an open swamp or a body 
of water is seen occasionally along the Michigan shore, as at 
Grand Haven. In no case, however, were any facts obtained 
that added anything essential to those given above. In con- 
cluding the section on dune encroachment, it may be said that 
the only conspicuous case of the survival of members of a pre- 
‘xisting flora is furnished by the swamps. It may at first seem 
‘surprising fact that the plants which are able to adapt them- 
selves to the intensely severe conditions of an advancing dune 
we hydrophytic willows and dogwoods, rather tian mesophytic 
daks, basswoods, and maples, or xerophytic pines. Such a view 
as this comes from a misconception of the change that is needed 
ithe life-habits of the plant. The relation to the soil water is 
ly altered, or at least not rapidly altered. hoi 4 
ee apa elongating stem makes it more and more nang 
the ag draw water from the original root coset . Me 
Pei z the willow, at any rate, this is Oe f aps 
the ties - of new roots along the buried stem, whic at 
tioned <n utilize the moisture in the sand. The ate zie 
least, ig za ~~ unable to send out such roots, and here, 
There : possible source of their failure. ake oe 
™ of ada te line along which the solution 0 ne is 
later — may be sought. A number of eco ie - 
Many swa, ave commented on the xerophytic oar et 
swamp es plants. These structures are not to be ee a 
Ss, but are particularly well-marked in pian 
