58 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
Cerastium iriviale^ var. holosieoides^ whose distribution seems to be 
connected with the tidal part of the river. The type or common 
form grows in all sorts of places, but this well-marked variety grows 
only in spots where it can be irrigated by the daily flow of the tide. 
Its upper limit is at about the top of the North Inch, but it does not 
descend to the saline parts of the river. This daily irrigation has an 
evident effect upon other plants, but this is the only one apparently 
whose range is distinctly affected by it. 
Whilst we have endeavoured to show how the flora of the tidal 
district is differentiated from that of the other two districts by the 
presence of certain plants, we have yet to see how it is characterised 
by the less abundance, or even the entire absence, of certain other 
plants. It must first be premised that there is no hard and first line 
of demarcation between any of the districts. Unless for the presence 
of tidal action in the river, the portion of the tidal district which lies 
above the Friarton has a good claim to be included in the middle, or 
Strathmore district, and hence some of the distinctive features of the 
flora of the two upper districts intrude into the lower district, and 
tend to lessen the differences between them. These differences con¬ 
sist chiefly in the presence of three groups of plants, viz. : — 
1. Alpine Plants. 
2. Alpine-maritime Plants. 
3. Plants which may perhaps be considered upland, or 
sub-alpine. 
Group I—iVlpine Plants.—No part of the Tay is rich in alpine 
plants, but three species are abundant and well established in the two 
upper districts and are found also in the uppermost portion of the 
third, but only in a more or less sporadic fashion. These are Alclie- 
milla alpina^ Saxifi'aga aizdides^ and Oxyria renifo 7 ' 7 nis. 
Group 2—The Alpine-maritime Plants ?x^Silenemaritima^Armeria 
maritima^ and Plafitago maritima, —Though we have termed these 
Alpine-maritime, the term is really erroneous, for, though the plants 
occur in suitable places on the sea-shore and on the mountains, it is 
neither the maritime nor the alpine nature of these localities which 
attracts them, but because it is there that they find less competition 
for room, and can flourish where some other plants cannot. This 
same freedom from overcrowding they find on the shingles and 
younger haughlands, and on such places they abound and form a 
conspicuous feature of the vegetation in the two upper districts, while 
from the greater part of the tidal district they are absent. 
Group 3—Upland or Sub-alpine Plants.—Whether the species to 
