240 
amount of heat available. This is little liable 
to vary in open meadows and pastures within 
several degrees of latitude. But in a wood 
the temperature is considerably lower than 
that of the surrounding open country. Heat 
and colour go together; hence it may be that 
there is an absence of colour in the wood- 
lands. 
Moreover, each plant requires a definite 
amount of heat before it will commence to 
flower and later ripen seed. If one excepts 
the bulbous plants that flower before the trees 
are in leaf, and the trees themselves, the 
generality of the woodland plants flower late, 
in spite of their usually perennial character. 
Temperature has also an effect upon the 
general conditions of plant-life, and this 
explains the absence of life (lower zones) in 
a cold dry wood. The absence of moisture 
with cold prevents the proper balancing of 
conditions for assimilation; respiration, tran- 
spiration, and osmosis are slow, 
Protection of the Woods.—The denseness, 
darkness, and coldness of woods generally 
are retarding factors which may be well 
compensated by another feature, and that 
is their protection, The association of the 
trees in a close formation, not only serves 
as a protection in itself to the tree unit, but 
it has a corresponding conservative effect 
upon the rest of the flora of a wood. The 
scrub layer and the ground flora are effectively 
protected. Wind erosion is almost minimized 
by the covering tree zone. The effect of frost 
is also greatly reduced. 
Trees further protect the soil from being 
worn away by the denuding effect of rain or 
hail. Where trees drip there is some local 
erosion, but this is restricted in its work, and 
the soil is not carried far away. 
In a wood, also, the effect of a drought is 
far less marked, though a clayey soil suffers 
more severely in this respect. The scorching 
heat of the sun in ordinary weather is again 
moderated by the tree zone. Hence the pro- 
tective effect of trees is, on the whole, decidedly 
advantageous to woodland plants. 
Wet and Dry Woods.— Whilst the character 
of the soil determines the type of woodland— 
there being five main types: pedunculate Oak, 
sessile Oak, Birch, Beech, Ash, with com- 
binations-—the water content of the soil has 
a good deal to do with tree dispersal, and also 
affects the scrub and ground flora. 
Thus a wet clay is characterized by the 
pedunculate Oak, whilst a dry, sandy soil is 
occupied by the sessile Oak. The extent of 
the effect of soil may be seen in the same 
tract of wood, for on the siliceous slates of the 
Charnwood Forest region, which give rise to 
HINTS AND NOTES 
a wet clay, Birch, which is a wet-soil type of 
tree, is found, with Oak encircling it where 
those rocks are in turn surrounded by the 
drier, more sandy red marl. 
The ground flora in a wet and dry wood 
will differ correspondingly, such plants as 
Bugle and Tussock Grass indicating a wet 
wood. The extreme type of wet wood is 
afforded by the Alder-Willow association, 
which is characteristic of marshy or aquatic 
plant formations. 
Effect of Tree-felling on Rainfall.—When 
trees are felled, not only is the shade which 
they afford at once lost and sunlight able 
therefore to penetrate near to the surface, 
but the removal of the trunk and branches, 
with the numerous leaves, causes the moisture 
which they accumulate to fall directly upon 
the earth. Here, on a porous surface, the 
water percolates and finds its way down to a 
subterranean reservoir. Water accumulated 
upon a clayey soil soon evaporates in the 
open. Radiation is more rapid over a tree- 
less area than in a forest area. 
The retention of the moisture by the indi- 
vidual trees may be, moreover, considered 
apart from the aggregate amount of moisture 
present in a forest, regarded as a unit in itself. 
The association of numbers of trees causes 
the atmosphere itself to remain charged with 
moisture, and evaporation is consequently 
slow. The preservation of moisture at the 
surface by a tree layer, and its retention by the 
lower strata of plants, are also features of a 
woodland area that must be considered in 
estimating the value of forests as water 
reservoirs. 
The retention of dew is also an important 
aspect. 
Effect of Woodlands upon Soil.— Perhaps the 
most outstanding feature of a wood or forest 
is the part it plays in the accumulation of 
organic matter, plant and animal, upon the 
surface, which in course of time becomes a 
valuable asset to the soil. This matter is 
known as humus, and it is to the presence 
of this in the soil that the woodland plants 
owe their distribution to a great extent. 
Whilst many plants that grow in a wood are 
able to exist in the open upon other soils, or 
those not rich (or even deficient) in humus, 
some that grow in the open do not care for 
humus. It is suggested that simple experi- 
ments be made in growing plants in soil with 
and without humus, and noting the effect. 
Since the original vegetation was woodland, 
it should be expected that the removal of this 
from a large area by disafforestation has been 
the cause of differentiation into meadow and 
pasture, heath, and other types of vegetation 
