1899] DUNE FLORAS OF LAKE MICHIGAN 177 
grow pari passu with the dune, and such an obstacle is furnished 
by anumber of perennial plants, which spread by means of rhi- 
zome propagation. These dune-forming plants must be peren- 
nials; otherwise the dune would be destroyed at the end of the 
growing season. Such annual dunes are very common on the 
beach, Clumps of Cakile have been seen, about which there is 
piled a miniature dune. In the same manner there may be 
formed biennial dunes about individual plants of Artemisia or 
Cnicus. A perennial dune, however, requires perennial dune- 
formers. A second necessity in a dune-former is the ability to 
Spread radially by rhizome propagation, for only in this way can 
the area of the obstacle and the area of the dune be enlarged. 
The only notable exception to this rule is found in the case 
of cottonwoods and such perennial grasses as Andropogon and 
rll these plants grow in groups or tufts and will be 
described later, . 
The plant which serves as an obstacle for the wind must also 
cc power of growing out into the light when buried by 
< nes This Pretty permits the rise of the dune in altitude, 
plants ap Propagation permits the increase in area. Most 
cause asia by reason of this third requirement, partly 
Partly be €y are unable to rise above the sand when buried, and 
ing y aia elongation increases the difficulty of draw- 
dune 09 cient supply of water from the soil. The roots of 
sand-blast « are often uncovered and exposed to wind and 
ction; hence plants unable to survive a period of root 
€XDos os 
Fea sre cannot live in such a habitat. In short, a successful 
'On to : 
the above particular requirements, a dune-former 
Must, of 
eXtreme alee be perfectly equipped with a set of the most 
ch is oP) tic adaptations. Nowhere else except on the 
€ , 
ve 80-great exposure to heat, cold, and wind. 
e 
fF ¥Pical and successful of all dune-forming plants 
