374 
Review: 
heath vegetation.” Besides, these factors there are, of course, the 
special chemical factors, such as the presence of common salt, which 
profoundly alters the vegetation, since only specially adapted plants 
which can tolerate it are able to exist at all. 
Dr. Graebner points out that since the existence of any plant- 
community in a given spot is due to the combined effect of a 
number of factors developed in different degrees, it is not easy to 
decide which to choose as a basis for primary classification. In the 
case of a country like Germany, however, where variations of climate 
are certainly not of the first importance, the problem is practically 
limited to choosing between the edaphic or soil factors. This, of 
course, is also the case in the British Isles. At first sight it would 
appear that the water of the soil—that fundamentally important 
factor in the life of a higher plant—is the natural factor on the 
variations of which we should base our primary classification, and 
this is the usual method adopted, for instance, by Warming in his 
earlier work. But Dr. Graebner is of opinion that this involves so 
many contradictions that such a principle does not, in fact, yield a 
natural classification. Thus the soil of an open heath and of a 
deciduous wood may possess the same water-content, and so may a Fen 
(Wiesenmoor) and a Sphagnum-moor (Hochmoor), and yet they are 
fundamentally different plant-communities, possessing different floras. 
The heath, the Sphagnum-moor and certain types of wood on the one 
hand have a whole series of characteristic species in common and so 
on the other have the open hills, rocks and other types of wood. It is 
true that Warming and others try to get over this difficulty by distin¬ 
guishing between “ physical ” and “ physiological ” soil-water, i.e., 
between the total water-content of the soil and that which is actually 
available for the roots of the plants, but although this distinction is 
of great importance, by no means all the anomalies of the classifi¬ 
cation by water-content are removed by its aid, nor do we know the 
facts accurately enough to employ it with precision. Dr. Graebner 
proposes to classify plant-communities on the basis of the total crop 
of vegetable material produced in the year, using this as an index 
of the “ fertility ” or “ poverty ” of the soil, itself due, as already 
shown, to very various factors. On the “ good ” soils the most 
luxuriant vegetation will flourish, excluding the slower growing, while 
on the “ poor ” soils the former will no longer be able to exist and 
will give place to a less exacting type of plant-community. 
The author thus divides the plant-communities of Germany into 
three great series : (i) those in which the physical and chemical 
conditions are favourable (apart from water-content); (ii) those in 
which the plants are only able to obtain a small amount of food 
from the soil, even in favourable seasons ; (iii) those on salty ground, 
where the conditions of life are of course quite special. These three 
series are then again divided according to the water-content of the 
soil. 
Roughly speaking the first series may be said to contain steppe 
(dry), natural meadow- and forest-vegetation (damp), as well as fen 
and most aquatic vegetation (wet), while the second is equivalent 
to the great heath-series, with the “ sandy plain ” (dry), the true 
“ heath ” (damp) and the “ Hochmoor ” (wet). 
As Dr. Graebner remarks, this method has at any rate the 
advantage of separating, in the first instance, series of formations 
which are very clearly distinct, both in type of vegetation 
