Trees.—Although trees are the highest form of vegetable life, they are in many ways more sensitive than bushy or grassy vegetation. They are sensitive to drought, especially when young. Everyone who has traveled from a well-watered region to one that is dry knows how the trees diminish in size and become 'scrubby, or else become few in number and are limited to places with more than the usual amount of moisture. Trees also need a fairly long growing season. That is why the tree-line on mountains is lower than the upper limit of grasses. On the higher slopes of the mountains, although there is plenty of moisture the warm period when growth is possible is not long enough to enable trees to make their growth and ripen their seeds, although grasses succeed Without difficulty. Accordingly trees attain a fine growth and form great forests in regions which have a moderately long warm season during which there is abundant moisture. Such regions may be as varied as the Belgian Congo, Si beria, the eastern United States, and the Andes of southern Chili.
Bushes and Scrub.—The plants classified under this head range all the way from scrubby trees to perennials with more or less woody stems which die back in part after each growing season. Such plants are mixed with the trees in most forested regions. Where the condi tions of climate or soil become unfavorable to trees, however, bushes crowd them out and become the chief growth. This can be seen near the tree line on the sides of mountains where low temperature pre vents the growth of trees sooner than of bushes. It is also seen on the edges of swamps where too much moisture in the soil prevents many kinds of trees from growing, but does not drive out the bushes. Again where the soil becomes thin and hence dry, the trees give place to bushes. The most noteworthy of all regions for bushes, however, are subtropical and desert regions or the parts of the torrid zone where the dry season is particularly long. The mountains of Sicily with their scrubby "dry forests," the sage brush desert of Utah, the bushy desert of Arizona, and the areas of tropical scrub in the dry parts of Colombia are all examples of this type.
The bushy growths of dry regions differ from trees in being able to maintain themselves through protracted droughts provided they have water at occasional intervals. Many of them have drought resistant leaves. In some, like the laurel and live oak, the leaves are hard and shiny; in others such as the sage, they are soft and furry. Both types have coverings that hinder evaporation and thus protect the plants during the long dry season. Many such plants also bear
prickly leaves or spines.. These incidentally protect the plants against the ravages of animals, but in most cases they originate through a pro gressive reduction in the plant's evaporating surface. The plants in which evaporation is restricted have the best chance of survival in the desert.
Grasses.—Tne modest grasses grcois under a greater variety of cir cumstances than do either trees or bushes. Many grasses can com plete their growth and mature their seeds in a few weeks so that a single shower in the desert may be enough for them. They often grow so rapidly that they can thrive on high mountains where the warm season is too short for trees or bushes. Hence above the tree line there are lofty green pastures or "alps" that furnish food for sheep and cattle in Switzerland, Norway, and elsewhere. Grasses can endure not only aridity, low temperature, and short seasons of growth, but also excessive moisture which would be fatal to trees and bushes. Hence the dripping hills of Ireland, Scotland, and Wales, and the soggy marshes of Holland are clothed with soft. lush grass which makes excellent pasture.
A Mountain Showing World-wide Types of Vegetation.—A most interesting illustration of the relation of climate to trees, bushes, and grasses is found on the western slope of the Sierra Nevada in Cali fornia. At the base the climate is so dry that the traveler finds the plain and the lower foothills clothed with grass which is green only a few weeks. A little higher up, as the rainfall increases, bushy vegetation of many sorts is met, including the wild lilac, the yucca, and the manzanita, with its smooth hornlike stems. The live-oak tree, with its hard, prickly leaves appears, and before one has climbed far he is in the midst of a dry forest, that is, one composed of drought resistant trees many of which are scrubby. At higher altitudes where there is abundant rain the dry forest is replaced by a wet forest of broad-leaved trees like the ordinary oak and sycamore. Then, where the air is colder, the type changes to coniferous forests of pines, giant sequoias, spruces, and firs. Next, where the growing season becomes too short for trees, one struggles through a dense thicket of bushes bent down by six months of snow each year. Finally, when these are left below, the open grassy slopes of the "alps" are reached, for here where the temperature is too low for woody plants the grasses thrive far better than they do in the dry, hot desert far below. Thus within a day's climb one may find illustrations of most of the world's main types of vegetation.