52 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
The occurrence, however, of certain plants {e.g. Fucus) in the lower 
part of the Tidal district points to a probable considerable salinity. 
Temperature. —This is highest on the lower course of the river, 
and, though the difference between that part and the upper districts 
is by no means great, it seems to be sufficient to affect the distribu¬ 
tion of certain plants. 
Rainfall. —This, on the other hand, increases as the river is 
ascended. It is doubtful, however, whether this difference is 
sufficiently great to have an appreciable influence on the flora. 
Surface Geology .—There is no doubt that this is a factor in the 
distribution, acting either by the structural character or by the 
chemical constituents. 
II.—THE RIVER IS THE CHIEF FACTOR IN THE PRESENT 
DISTRIBUTION OF THE FLORA. 
In this paper we are not concerned with the origin of the flora in 
the county as a whole, but merely with the causes of its present 
distribution on the banks of the river. The chief factor in this 
distribution is the river itself, and its action is twofold in its nature. 
In the first place, it is the great medium of transportation, and in the 
second, it provides suitable habitats for the plants. 
It will be remembered that, in describing the characteristics of the 
sections into which we have divided the river, stress was laid upon 
the rapidity of the curre 7 it and the nature of the bed. By the rapi¬ 
dity of the current, not only are the seeds of plants carried from one 
part of the river to another, but in times of flood many plants are 
transported bodily. 
In estimating the effects of this carrying power we must take into 
consideration, not only the stretch of the river to which these papers 
is restricted, but the portion above our upper limit, and also the 
various affluents. Keeping this in view, it will be seen that the river 
can draw supplies of plants from a very large area. That area is, 
however, possibly not altogether co-extensive with the drainage basin 
of the Tay. The large lochs through which several of the rivers have 
to pass probably intercept, to a certain extent, the supply of seeds, 
&c., from the parts beyond them, and, if they do not altogether 
prove a barrier, at any rate retard the passage. It is perhaps from 
this cause that a greater number of species of alpine plants do not 
occur as casuals on the banks of the Tay. The slow-flowing affluents 
with muddy beds, as the Isla and the Earn, in like manner, are not 
so prolific in supplying the Tay as the more rapid tributaries. 
But the carrying power of the river would not avail much in 
making a rich flora, unless suitable habitats and sustenance were pro- 
