Review : 
373 
GERMAN VEGETATION. 
Die Pflanzenwelt Deutschlands, Lehrbuch der Formations- 
biologie, eine Darstellung der Lebensgeschichte der wildwachsenden 
Pflanzenvereine und der Kulturflachen, von Dr. Paul Graebner. 
Leipsig, 1909, Verlag von Quelle & Meyer, Pp. XI. and 374. Price 
7 marks. 
This very readable account of the natural plant-communities 
of Germany, by Dr. Graebner, already well-known as the author of 
“Die Heide Norddeutschlands ”—one of the most thorough of 
existing ecological studies of a single great plant-formation—and 
as the collaborator with Professor Ascherson in the standard 
“ Mitteleuropaischer Flora,” will be of particular interest to British 
students of vegetation, since it furnishes one of the first opportunities 
of systematically comparing the plant-communities of a neighbouring 
country with those of our own. 
In laying down his principles for the classification of plant- 
communities, Dr. Graebner sets out from those which are developed 
under the most favourable conditions of climate and soil, namely 
the rain-forests of the tropics, which produce the greatest annual 
“ crop ” of vegetable material in the world. Among these conditions 
—besides the well-known factors of continuous warmth, abundant 
and distributed rain-fall, etc.—he lays stress on the poverty or absence 
of humus (owing to the rapid oxidation of the fallen leaves and 
branches) and the consequent free aeration of the soil. Starting with 
the ideal growth-conditions of the tropical rain-forest he traces the 
gradual incoming and increasing dominance of factors adverse to 
vegetation as we pass on the one hand into regions with well-marked 
dry periods and thence into the sub-tropical deserts, and on the 
other into the colder temperate and arctic regions, where drought 
and winter, respectively, impose upon plants a longer and longer 
resting period. This progressive diminution of vegetative activity 
is accompanied, of course, by a progressive decrease of the annual 
“ crop ” of material produced. The responses of plant-form and 
life-history to these great climatic factors are briefly but skilfully 
expounded, and the author then proceeds to consider the other 
class of ecological factors—those connected with soil. Here also 
the appearance of adverse conditions is accompanied by a decrease 
in the mass of material annually produced. On “ good ” soil a 
large crop is produced, on “poor” soil a small one. But the 
“ goodness ” of a soil does not depend only on the amount of 
available mineral food, it is also greatly influenced by various other 
physical and chemical factors :—texture, air- and water-content, 
presence of humus and the like. Where humus, especially 
“ Rohhumus ” (dry peat), is formed on the surface, aeration of the 
soil is so much hindered that the plants are all shallow-rooting, and 
the same effect is produced by the presence of a hard layer fairly 
close to the surface, which physically prevents the roots penetrating 
to deeper layers. This hardening of the surface or limitation of the 
depth of the soil has a most important effect on the plant-covering, 
excluding many species altogether, and, in general, leading to a 
dwarfing of the vegetation ; so that, as the author remarks, “the 
same soil under the same conditions of moisture and warmth may 
bear perhaps in one place the finest high forest, in another only 
