6 Forms of Trees. [zoe 



oaks with lafge crowns, spreading- branches and repeatedly divided 

 trunks. The pines also, like P. Sabiniayia, are characterised by 

 their forked trunks, their upright limbs, and by their g-eneral 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 

 Lamberiiana^contorta, yeffreyi, ?i\so in Sequoia giganiea, 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 ph}'- 

 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- 



