DUNE FLORAS OF LAKE MICHIGAN 297 
1899] 
that these species are naturally adapted to a xerophytic life, and 
that, when the occasion arises, still further xerophytic conditions 
can be met successfully. 
The success of the willows and dogwoods on the dunes may 
bedue, in part, to yet another characteristic. It is well known 
that swamp plants are provided with extensive adaptations to 
Momoteaeration. This need is especially apparent in undrained 
‘amps, where the gases necessary for the underground tissues 
and organs have to be almost entirely supplied from above the 
surface of the soil. It is in these undrained swamps, too, that 
the accumulation of peat is so rapid. It seems rational, then, 
suppose that tenants of undrained swamps, by adapting them- 
selves to prevent suffocation, have also adapted themselves to 
withstand burial by sand without injury. Just what is the cause 
of death, when plants are partially buried by the soil, is, so far 
a the author knows, an unanswered question. A wide field for 
‘atomical study and physiological experiment lies open along 
this line. In the meantime the notion that plants of undrained 
‘amps are better fitted to suffer partial burial than are other 
Plants may remain as a tentative theory. 
in Considering the formation of secondary embryonic dunes, 
mention was made of Potentilla Anserina, Cephalanthus occidentalis, 
and Polygonuyy Hartwrightii as dune-formers under certain condi- 
ce Potentilla and Polygonum are extensively creeping 
- ag Cephalanthus is an erect shrub. All three are 
m 
pera naturally, and yet able, as has been said, to build 
ase pF a slow growth. In like manner they sometimes 
them Sa for a time when a wandering dune encroaches A 
is tlatively., are especially plastic where the agate of a se 
‘he most ee Among these plants Potentilla seems iY e 
that ee Pranic ” dune conditions. Another seb : 
anum, es ng degree of plasticity is Ligpercam a sat 
© dune 's shrub is very common in the undrained ec : 
‘dvancin ae and very often finds itself in the path are 
fringe . ‘ne. Hypericum, like Salix, often forms a marg 
ap Swamp, and miniature lines of this shrub are 
low d 
