268 BOTANICAL GAZETTE [octoper 
society. But the lichens of the trees 150™ away are on the whole of a 
type very sparingly represented on the riprap, those of the trees being 
of the genera mentioned above and very seldom seen on the riprap, 
and in the main then by different species, while those of the riprap 
society are mainly a number of crustose species absent from the trees 
or only sparingly represented on them, and several fruticose species 
wholly absent from the trees. All of this will appear more plainly 
after a list of species of the society under consideration is given. The 
causes which have led to the possession of the riprap by certain 
types of lichens may also be discussed to better advantage later, the 
intention here being merely to bring out the fact that proximity has 
not enabled the lichens of the trees to gain possession of the rocks 
in face of certain unfavorable conditions, and that other types have 
consequently gained the ascendancy. 
As to moisture, the swamp brings an abundance of soil moisture, 
especially toward the lower ends of the riprap extensions. Thus at 
certain times, as in wet seasons and after rains, the fruticose Clad: 
onias grow well in the somewhat shaded and moist openings between 
the blocks of riprap, and pass into the desiccated condition without 
injury whenever the moisture becomes deficient. Since the soil 
moisture does not pass upward through the loose riprap to any great 
extent, and the small blocks retain very little moisture, the upp 
surface of the sandstone blocks becomes drier than would the uppét 
surface of a similarly exposed solid wall or natural exposure of the 
same kind of rock. Accordingly the conditions on the upper surfaces 
of the extensions are quite xerophytic, especially toward the uppe 
portion of each extension where farthest removed from the PS 
and where the vertical height of the extensions averages about gt 
Passing downward on the upper surface of the extensions, as 
height of each one gradually decreases and the soil becomes ee 
moist, the soil moisture works upward through the riprap i 
more and more, so that the conditions become gradually less * 
phytic. Passing to other considerations for the present, the ¢ i 
in lichen species upon the upper surface, resulting from the ¥ 
ecologic conditions, will receive attention below- ae society 
The conditions determining the composition of the ere 
are plainly quite different, surely drier and doubtless 0? the: 
