202 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
plants, but the group includes those sea-weeds whose zones of 
vegetation are so well indicated in descending from high to low 
water-mark on rocky shores, e.g., Enteromorpha^ species of Fucus^ 
and Laminaria. 
XII. —Halophytes, or salt-loving plants, such as A7}wiophila.^ 
E/y/nus, Saiicornia, Triglochin maritinitim.^ etc. 
These groups include all the social species of any importance in 
our district. In this paper I am to consider only those of greatest 
interest, either from the part they play in the landscape, or from their 
value with regard to the sub-division of the Tay basin into natural 
botanical regions. 
tree associations. 
The leading part which trees play in the scenery of a country is 
evident. The class as a whole is a social one, thriving best when its 
members grow side by side, close enough to resist the sweep of the 
wind, and to retain the amount of moisture necessary for their 
growth. But only a few of the class are social species. Many, such 
as the ash, elm, sycamore, poplar, and rowan, are, as a rule, sparsely 
mixed through woods where beech, oak, pine, and birch may 
dominate. 
The Beech, Fagus syivaiica, Linn.—The beech is essentially a 
social tree. In Scotland it is an introduced element planted by man, 
but where it finds a suitable home in our carse-lands and plains it 
tends to drive out the other species planted with it. In walking 
through a crowded wood of mixed trees, one sees on looking up at 
the “ crowns ” that the huge outgrowing branches of the beeches are 
overshadowing and crowding the neighbouring species. 
The beech is a shade-giving species to a greater degree than 
almost any other of our common wood trees. One might arrange 
the trees in the order of the shade they afford, beginning with those 
giving least, as follows :—birch, larch, Scots pine, ash, elm, oak, 
beech, and spruce. The greatest gap in the series would be between 
oak and beech. 
Because of the deep shade, plants grow with difficulty under 
beech trees. Seedlings of most other trees grow poorly, if at all, 
although beech seedlings grow well. In the darker parts of the 
wood the flora is very poor, only a few grasses and mosses prevailing. 
But where the trees are less crowded, or are mixed here and there 
with other species, so that more light enters, a richer association of 
plants springs up. 
In this habitat, where moisture, shade, and richness of humus 
