Fig. 323.—Salix: male flowers of S. pentandra (a), S. aurita (b), S. rubra (c), female flowers of S. aurita (d), S. nigricans (e), S. mollissima (f).
Salix (Willow) has short-stalked, most frequently lanceolate leaves and erect catkins with undivided bracts (Fig. [322]). The flowers are naked; 1 (o in a-f) or 2 yellowish glands situated in the median line. In the ♂-flower generally two stamens, situated laterally like the carpels in the ♀-flower. Various forms are seen in Fig. [323].—The terminal bud of the branches often aborts regularly, the uppermost lateral bud taking its place.
Populus (Aspen, Poplar) has long-stalked, more or less round or cordate leaves with drawn-out apex; catkin pendulous; lobed bracts; perianth cup-like with oblique edge; stamens usually numerous; stigmas often divided.—P. tremula (Aspen) has received its name from the tremor of the leaves: cf. “to shake like an aspen leaf.”
Pollination. The Poplars are wind-pollinated. The Willows have sticky pollen and are pollinated by insects. The catkins of the Willows, especially the ♂, are more conspicuous, from the numerous, closely-packed, yellow flowers, rich in honey and pollen. The catkins often appear before the foliage and so are much more easily seen, whilst at this time of the year the number of competing honey-flowers is smaller, and the insect visits consequently more numerous. On many catkins of the Willow the flowers open earliest on the side which is turned towards the sun and in descending order, i.e. the upper flowers develop before the lower ones. Hybrids frequently appear.
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.”