1899] DUNE FLORAS OF LAKE MICHIGAN 285 
slowly. This dune, however, is at the upper end of the curved 
wind-sweep previously mentioned, so that northwest winds con- 
tribute to its advance. Thus it becomes possible for sha duce 
i to cause the advance of two dunes toward each other and 
ee a, of a flora. The advance of all dunes at this 
itil : is shown by the comparatively abun- 
arelict of the past a seatteies bn This vegetation is not 
ill of the Fst on Bia git Se completely Agstoy 
years, unless Dadiions aa. hee eS 2 : ee ras 
surface by which one ma be si : aan - : a 
plant society. 7 rpret the history of this dying 
ul oe Sah - - dune upon a forest is shown in figs. 
the scrub sine, eendes this vicinity consist principally of 
cinea tinctoria, eittier: ras ‘i a gi one 
degree of es an th ese trees can survive any such 
succumbs long before th e€ cottonwood. The oak, especially, 
along the dune ae ne eaeies tree is buried; the dead trees 
@pine that is half ie . both pictures are oaks. Fig. 12 shows 
The dead trees in uried, but apparently as vigorous as ever. 
they seem im ee Z5 and 76 are mostly scrub pines, and 
foundings than d hed greater adaptation to their new sur- 
of individual as oaks. There appears to bea wide range 
a the ae in pine trees; some dying almost as soon 
tant as the cottonwo is ay ott while others are nearly as re 
i that the tree is = a ‘a both pines and oaks the first obvious 
Wing trees along th Pets losing struggle is etiolation. The 
4 dark green a. € margin (as in figs. zz and 72) rarely have 
Steen, and in a. In most cases the leaves are yellowish 
‘merous or large cases almost white. Nor are the leaves as 
Such tree ‘e as on healthy trees. 
foomed to ty ups, as are shown in jigs. 9, 10, 73: and 14, are 
them in the _ death. The length of life allotted to 
dune’s advan ure depends almost entirely upon the rate of the 
that an aii, There are some evidences in favor of the view 
1dual pine tree can endure a deeper covering before 
