56 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
younger riverine deposits, such as the low haughlands, stanners, back¬ 
waters, marshlands, and foreshores. 
2. For the formation and stocking with plants of these deposits 
a rapid current is necessary. 
3. Therefore the richness of the flora is determined by the rapidity 
of the flow of the river. 
III.-CHARACTERISTICS OF THE DISTRICT FLORAS. 
From what has been already said regarding the physical characters 
of our three districts it may be readily conjectured that, while there 
may be differences between the floras of all three, yet the tidal 
district will present a great divergence from the other two districts 
than they do from each other. The action of the tides, the proximity 
to the sea, and the somewhat higher temperature, combine, as has 
been already suggested, and as we shall try to show, to bring about 
this result. 
The native flora of the tidal district seems capable of being 
divided into the following groups ;— 
1. Maritime Plants. 
2. Plants whose distribution seems to be governed by the 
temperature. 
3. Plants dependent on the action of the tides. 
4. Plants which it has in common with the other two districts. 
These constitute a majority of the flora. 
Group I—The Maritime Plants.—Of this group there are 
apparently two classes, one containing the plants whose distribution 
is co-extensive with a certain degree of salinity in the river; the 
other, those which, while most abundant when the water is salt, yet 
have an extension of their range to places where there is reported to 
be no salinity. 
Is salt therefore essential to the growth and wellbeing of some of 
these plants? This question, as regards the majority of our maritime 
plants, must, we think, be answered in the negative. We have grown 
some of them (as Glaiiso maritwid) in ordinary garden soil, and with¬ 
out any superfluity of moisture, and have found that they did very 
well. It is clear, therefore, that in this case the presence of salt is 
not essential, but experiment can alone show whether this holds good 
with all the species. On the other hand, as the range of some of 
the plants is co-extensive with the presence of salt, there must be a 
reason for this connection, and it seems to be this. Many common 
plants cannot grow in the more or less saline places where the mari¬ 
time plants flourish. There is, therefore, less competition for room 
in these places, and less of a struggle for existence, and in the struggle 
