180 Proceedings of the Koyal Society of Edinburgh. [Sess. 
them to adapt themselves to the various combinations of edaphic and 
climatic conditions, is so great that there are comparatively few spots, 
where existence is possible, in which some plant or other is not able to 
thrive and carry on its metabolic activities. Now in order to maintain a 
proper tone of health a plant has of necessity to respond in suitable ways 
to all the varying external impressions. A plant is therefore in a constant 
and continual state of change, owing to the never-ceasing mechanical, 
physical, and chemical changes of its unstable environment. The plastic 
nature of many plants enables them to modify their organs in reciprocation 
to any fairly constant set of environmental conditions, and it is in this 
endeavour to accommodate themselves for the maintenance of healthy exist- 
ence in places that are either inhospitable or too luxurious, that certain 
deviations, either fixed or transient, from the usual forms of more normal 
environments are to be accounted for, and such variations occur in almost 
every loch. That some of such variants may doubtless be concerned in 
the origin of new species and varieties is the impression that I have 
received, but I hasten to add that other causes also contribute towards 
that process. 
The rapid increase of aquatic and marsh plants in reservoirs that are 
used for the public water supply is occasionally a matter of anxiety and 
expense to the owners. Enormous sums of money are frequently paid by 
public bodies for advice respecting the construction of reservoirs to persons 
wholly unacquainted with the local geological features, as well as with the 
flora and fauna of the district. Whilst it is very unwise to construct a 
reservoir over a geological fault and expect it to hold water (and this has 
been done), it is equally vain to make a shallow reservoir in the line of the 
constant migration of water-fowl (i.e. between their resorts), and expect 
it to maintain a freedom from water plants. By consulting the table on 
pp. 97-99 it will be seen that the greatest depth at which aquatic plants 
will flourish in Scottish waters is about 40 feet. It is very unlikely, 
however, that the species capable of growing at such a depth will ever 
become a nuisance in a reservoir. But at a depth of 20 feet it will be 
found that, in suitable water, many species capable of giving trouble will 
flourish. Upon consideration of these facts, it seems advisable, as a 
prevention against the development of water plants, to construct reservoirs 
with sides so steep that a minimum depth of from 20 to 25 feet will be 
maintained within a few yards of the margin. Moreover, the sides, unless 
of natural rock, should be faced with stonework, which will further impede 
the growth of plants, as well as prevent discoloration of the water by 
wave-erosion. 
