There are about 180 species existing in the northern, cold and temperate latitudes. Some in the Polar regions are scarcely more than an inch in height, and have a creeping rhizome (_Salix herbacea_, _polaris_, _reticulata_). Fossil forms are found in the Tertiary and perhaps also in the Upper Cretaceous. USES. Principally for ornamental trees, as they grow very quickly and are easily propagated by cuttings, _S. babylonica_, Weeping Willow; _S. purpurea_; _Populus alba_, Silver Poplar; _P. pyramidalis_, Pyramid Poplar--a form of _P. nigra_; _P. monilifera_, Canadian Poplar. The wood is very poor and little used; the branches of many Willows are cultivated for basket-making, etc. The wood of the Aspen is used for matches. The bark contains tannin and, in many Willows, a very bitter extract, _Salicin_ (_S. pentandra_, _fragilis_). Salicylic acid (officinal) is obtained from _Salix_. Balsam is extracted from the buds of many Poplars, especially when the leaves are shooting. Family 2. =Casuarinifloræ.= Trees with verticillate, scale-like leaves forming sheaths at the nodes. Monœcious. Flowers unisexual. ♂-flowers in catkins; ♀ in short spikes. _Pollen-tube entering the ovule at the chalaza_, and not through the micropyle. Ovary 1-seeded, unilocular. Carpels uniting into a multiple fruit. Only one order. Order. =Casuarinaceæ.= Trees (30 species), from Australia and certain parts of S.E. Asia, with peculiar, equisetum-like appearance. The leaves are verticillate, scale-like and united into sheaths. The internodes are furrowed. Branching verticillate. The unisexual flowers are situated in catkins or short spikes. The ♂-flower has a central stamen, surrounded by 2 median, scale-like perianth-leaves and 2 lateral bracteoles. The ♀-flower has a 1-chambered ovary (2 ascending, orthotropous ovules), no perianth, but 2 large, lateral bracteoles which finally become woody and form two valves, between which the nut-like fruit is situated. The multiple-fruits therefore resemble small cones.--_Casuarina equisetifolia_, cultivated, gives “iron-wood.”