6 forms of Trees. [ZOE 
oaks with large crowns, spreading branches and repeatedly divided 
trunks. The pines also, like P. Sabiniana, are characterised by 
their forked trunks, their upright limbs, and by their general re- 
semblance to deciduous trees. As regards shrubs of all kinds, they 
are hardly less influenced by snowfall. In the snow-visited forests 
at least, the evergreen shrubs show a low depressed form, sometimes 
spreading out like dishes on the ground. Other species, again, like 
the manzanitas, possess repeatedly zig-zag bent limbs especially 
adapted to resist the pressure of snow and wind. Such zig-zag 
form is also possessed by the branches of trees, greatly assisting 
them to resist outside pressure of any kind. Thus while the lower 
or central branches of most of the pines in the snow region slope 
‘downwards, the upper limbs, which are naturally less exposed to 
Snow pressure, assume a horizontal position, but are compensated 
by being repeatedly bent and furnished with heavy knees. Such 
limbs are generally seen in the various species of pines, such as 
Lambertiana, contorta, Feffreyé, also in Seguoia Sigantea, etc., while 
they are almost absent in the spruces and firs, the sloping elastic 
limbs of which continue to the tops. 
SUNLIGHT AND HEAT. 
Another important agency in shaping the forms of trees is the 
direct sunlight and heat. As the force of the direct rays of the sun . 
is different in different places, it follows that their effect upon trees 
and shrubs must vary with the locality, as well as with the phy- 
siological structure and nature of the plants. Various other agencies, 
such as the moisture in the air, the force of the wind, the rainfall, 
dews and fogs, combine with the sunlight and heat, either in decreas. 
ing or increasing the effects. It is especially in warm and dry 
regions where the heat and light are all powerful in modifying and 
directing the development of the form of a tree or shrub. An ex- 
cess of heat and light is nearly always hurtful and may even be so 
injurious as to kill the trees, or make them unfit for the region. It 
is especially the horticulturist that notes these effects of heat and 
light. In tender plants the effects are more pronounced and prin- 
cipally of two kinds. The direct rays of the sun injure the stem 
or trunk on the southwest side, or on the side on which the greatest 
force of the sun rays are concentrated during or shortly after mid- 
day. The tender bark and cambium are scorched, dry up and pre- 
