Order. =Salicaceæ= (=Willows=). Trees with simple, scattered, _stipulate leaves_. _Diœcious_. The flowers are arranged in _simple inflorescences_ (spikes or racemes) which are termed catkins, and which fall off as a whole after flowering (♂) or after the ripening of the fruit (♀) (Fig. 322). The perianth is very imperfect[33] or wanting, particularly in _Salix_ (Fig. 323 _o_); the ♂-flower with 2–several stamens and without any trace of a carpel (_a_, _b_, _c_): the ♀-flower has a free bicarpellate ovary, _unilocular_, and formed from 2 lateral carpels with 2 _parietal_ (_median_) _placentæ_ and generally ∞ ovules; the style divides into two stigmas (_d_, _e_, _f_). The fruit is a two-valved _capsule_ and the very small seeds bear a _tuft of hairs_ at the base. _Endosperm absent._--~The catkins are situated on dwarf-branches, which in some species often develop before the leaves and bear at their base only scale-leaves; in others foliage-leaves are borne beneath the catkins. The vegetative bud commences with 2 bud-scales which are united on the anterior side into a scale. The capsule opens by the dorsal suture. The seed-hairs spring from the funicle.~ [Illustration: 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.