166 THE FEESH- WATER LOCHS OF SCOTLAND 
The mud occurring in peaty lochs is seldom of the black, evil- 
smelling kind, such as is commonly found in non-peaty lochs. This 
will be explained subsequently (p. 215). 
Many plants, e.g. Phragmites communis, Sparganium ramosum, 
Alisma Plantago, etc., always grow more luxuriantly when the mud is 
black and fetid ; but other plants, e.g. various species of Sphagnum, 
Isoetes lacustris. Lobelia Dortmanna, etc., are unable to endure that 
kind of mud, not directly because of its presence, but because other 
factors, e.g. difference in food-salts, are correlated with the presence of 
this or that kind of mud. A number of other plants are comparatively 
indifferent, e.g. Castalia speciosa, Menyanthes trifoliata, Carex rostrata, 
etc. It would be interesting to grow aquatic plants artificially in 
oecological conditions opposed to their usual natural habitat, and to 
study the results. 
From the foregoing and subsequent statements it will be readily 
understood that the flora of the lochs is subjected to many varying con- 
ditions. 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 existence in inhospitable places, that certain deviations from 
the normal forms of more kindly environments are to be accounted 
for. That such forms should receive definite specific, or in some 
cases even varietal, names is open to grave doubt. Physiologists 
and experimental botanists are becoming more and more sympathetic 
towards the simplicity of the astute George Bentham ; and whilst 
recognising, as did Bentham, the numerous forms fixed and transient, 
such are regarded as unit forms in the phylogenesis, or in the retro- 
gression of a species. Owing to the variability of individuals, a 
species is sometimes held to consist of an aggregate of various forms 
or units which deviate more or less from a type form, i.e. from the 
species proper. Such unit forms some botanists elevate to the rank 
of varieties and even species. In the British Flora by George 
Bentham, 5th ed., 1887, there are, for example, seven species of 
Hieracium ; whilst in the Manual of Biitish Botany by Charles 
Babington, 9th ed., 1904, by H. and J. Groves, almost the same 
material is made to yield ninety-seven species, besides numerous 
named varieties. To such extreme tenuity have the diagnoses of these 
variable plants been drawn that the most learned authorities are 
often unable to distinguish the different species by one another's 
