Trees and shrubs, which, in the structure of the vegetative shoot and
the catkin-like inflorescences, resemble the Quercifloræ, but the
structure of the flower differs so much from them, that the only order
brought under this heading--_Salicaceæ_--well deserves to be separated
and to form a family of its own, the nearest relatives of which are
still doubtful. ~As Juglandaceæ and Myricaceæ also deserve to be placed
in a special family, the name _Amentaceæ_ (_Catkin-bearers_), hitherto
applied to all of these plants, cannot be retained as the name of a
family.~
[Illustration: FIG. 322.--Male and female catkins of _Salix
caprea_.]
There is only one order.
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.
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.