180 Proceedings of the Royal 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 tlie 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 siich \'ariations 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 supplj?^ 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 re.sorts), 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 unlikel}^, 

 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, uidess 

 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. 



