The species most liable to be struck are oaks, poplars and pear trees; beech trees are exceptionally safe.
The poplars are almost entirely confined to the north temperate zone, but a few approach or even pass its northern limit, and they are widely distributed within that area; they show, like the willows, a partiality for moist ground and often line the river-sides in otherwise treeless districts.
As in all poplars, the catkins expand in early spring, long before the leaves unfold; the ovaries bear four linear stigma lobes; the capsules ripen in May.
In the Mountain Region at the bases of the mountains are oaks, hickories, chestnuts and white poplars: above these are hemlocks, beeches, birches, elms, ashes, maples and limes; and still higher up are spruce, white pine and balsam; and all but a comparatively few of the higher mountains are forest-clad to their summits.
The materials made use of are driftwood, green willows, birch and poplars; also mud and stones intermixed in such a manner as contributes to the strength of the dam; but there is no particular method observed, except that the work is carried on with a regular sweep, and that all the parts are made of equal strength.
Poplars grow in the valleys, and the cactus and sage brush are common.
Near the town was the famous fountain of Sauros, inclosed by fruit-bearing poplars; and not far from this was another spring, overhung by an evergreen plane tree which in popular belief marked the scene of the amours of Zeus and Europa.
Among trees, stunted pines, dwarf oaks, poplars, willows and the cypress are fairly plentiful.
Here and there one sees a solitary sifsaf tree, or a small plantation of poplars or white mulberries, which trees, with the date-palm, constitute the only timber of the country.
The Russian city, to the south-east, dating from 1865, has clean, broad streets lined with poplars, and canals, the low, pleasant-looking houses being surrounded by gardens.
Leaf-buds occasionally arise from the roots, when they are called adventitious; this occurs in many fruit trees, poplars, elms and others.
On the plateau willows, poplars and chestnut trees grow near the streams, but nine-tenths of the country is treeless, except for scrub.
Elms, limes and poplars are common north of the Tagus, ilexes, araucarias, myrtles, magnolias and a great variety of conifers in all parts.
Willows and poplars, with a few other plants of more temperate regions, are found rarely at Aix, and seemingly point to casual introduction from surrounding mountains.