300 BOTANICAL GAZETTE [APRIL 
4. Capture of the dune-complex by vegetation. 
The capture of a dune by plants may begin within the dune- 
complex itself oralongitsmargin. In either case the first appear: 
ance of the advancing vegetation is commonly in the lower 
places toward the water level. The reasons for this fact are 
obvious. These low places are well protected from the wind; 
there is no danger, therefore, of any sand-blast action on the 
plant organs nor any removal of soil from around the roots. 
When the growth begins at the foot of an advancing lee slope, 
there is, however, considerable likelihood that the plants will be 
covered by the sand. It is this fact which prevents the capture 
of a rapidly advancing dune; the vertical growth of the plant 
must be greater than the vertical component of the dunes 
advance. ‘ 
The most important reason for the first appearance of plants 
at lower levels is the soil moisture. It is the moisture at the 
surface of the soil which causes to a large degree the lodgment 
of seeds, and especially light cottony seeds like those of the cot- 
tonwood and willow. At the base of the dune shown in fig: % 
where it is encroaching on a swamp pool, there is a line of young 
cottonwoods and willows several inches above the level her 
pool. The seeds were blown across the complex by the winds 
when they reached the crest of the advancing lee slope, they 
rolled down to the base together with the sand. The base f re 
dune is always moist several inches above the surface 0 
water because of capillarity. As soon as the sand yen 
reached the moist soil near the base, the movement was chec 
and both found lodgment. cottom 
The moisture necessary for the germination of the vere 
wood and willow seeds is also furnished at these ae eo 
near the water level. The danger of being covered ne eee 
ing sand is much less at this place because moist sam collet 
compact than Jry sand, and because moist sand does penn 
about the growing plants. The dune shown in the for sat likely 
jiz. 9, however, is advancing very rapidly, and sai . to pre 
that the growth of the young plants will be rapid enoug 
