JANUARY, 1902. BRUNCKEN— STUDIES IN PLANT DISTRIBUTION. 
27 
It seems to me that the similarity consists in this : In the dry- 
places, especially the gravelly, strongly insolated slopes, there 
is very little humus. What little there may be found is some- 
times sour or "raw" ; in fact the dense tangle which roots often 
form in such localities is well known for its tendency to produce 
sourness by hindering aeration. Similar sour humus is found in 
the wet, swampy forests. Sour humus makes it more difficult for 
the roots to absorb moisture, and consequently it becomes neces- 
sary for the plant to reduce the transpiration. The lack of oxy- 
gen and assimilable forms of nitrogen in such soils still further 
induces a weakening of physiological activity. Now the series of 
Viola species outlined above shows clearly that the number and 
importance of xerophilous adaptations increases in inverse pro- 
portion of the presence of fresh, w'ell-aerated humus, from the 
maple-beech forests to the dry hillsides on the one hand and the 
boggy elm and black ash forests on the other. Does not this 
circumstance indicate that there must be some causal connection 
between the quality of the humus and the occurrence of xerophy- 
tic characters ? 
It will be noticed, however, that there is a marked difference 
in the kind of adaptation between the dry and wet xerophytic sta- 
tions. In the dry localities the most conspicuous feature is hairi- 
ness and next to it the rolling in of the leaf edge. In the swamp, 
what one notices most of all is the reduction in size, both of the 
plant as a whole and the proportion of the various organs of as- 
similation and transpiration. V. Lahradorica is one of the small- 
est of our violets, both as regards length of stems and breadth of 
leaves. (The flowers are also smaller than in most other species.) 
V. hlanda is smaller still. European species in similar stations, 
like V. palustris, which grows in sphagnum bogs, and V. hiHora, 
which is found in gulches and on rocks where the water is con- 
stantly dripping, are also among the smallest of violets. On the 
other hand, V. sororia does not appear a bit less vigorous and full- 
sized than its cousin V. ohliqiia, though the latter has far better 
advantages as regards humus. 
It will suggest itself therefore that the plants growing in 
sour humus have a tendency towards reduction in size, and a 
glance at other species than those of the genus Viola confirms this 
impression. Although hairiness is not entirely absent in bog 
plants, it is uncommon (8) ; but nearly all such species are dis- 
tinguished by small leaves. This is true of many Ericaceae, and 
of conifers, such as tamarack and spruce. In very bad localities, 
such as sphagnum bogs, the spruce which occasionally grow there 
(8> See Warming, Op. cit., page 174. 
