_Æsculus_ (Horse-Chestnut). Trees with opposite, digitate, dentate
leaves without stipules; the inflorescence is composed of unipared
scorpioid cymes arranged in a pyramidal panicle (termed a thyrsus). The
flowers are irregular, with an _oblique plane of symmetry_ (through the
4th sepal, Fig. 471); there are 5 sepals, 5 free petals, of which the
one lying between S^3 and S^5 is the smallest (see Fig. 471) and may
be absent; stamens 7 (5 + 2), three being suppressed; gynœceum simple,
3-carpellary and 3-locular, with single style; of the two ovules one is
ascending, the other descending (Fig. 472).--The fruit is a 3-valvate,
sometimes spiny, capsule, with loculicidal dehiscence, the seed having
a large hilum, a curved embryo without endosperm and united cotyledons
(the radicle lies in a fold of the testa, Fig. 473). _Æ. hippocastanum_
(Greece, Asia), introduced into cultivation about 300 years ago;
the majority of the other species, _e.g. Æ. pavia_, etc., several
of which are frequently cultivated in gardens, are from N. America.
~The flower of the Horse-Chestnut is adapted for bees, whose abdomen
touches the anthers or style when visiting the flower. The flowers are
protogynous.~
The other Sapindaceæ have most frequently 4 sepals, 8 stamens,
various fruits (septicidal capsule, nuts with or without wings,
schizocarp), etc. _Serjania_, _Cardiospermum_, _Sapindus_,
_Koelreuteria_, etc. (about 118 genera, 970 species). The seeds
of _Paullinia sorbilis_ contain caffeine, and are used as “Pasta
guaranà,” in the North Western Brazils in the manufacture of a
common drink. _Nephelium_ (or _Euphoria_) _litchi_ (with edible
aril), and other species, from Asia.
[Illustration: FIG. 474.--Samara of _Acer platanoides_.]
Order 4. =Aceraceæ.= This order is so closely allied to the Sapindaceæ,
that some authorities have classed it with them. The main difference
is in the _regularity_ of the flowers, and the =2=-merous gynœceum
(in abnormal cases several carpels occur).--They are trees, and, like
the Horse-Chestnuts, have opposite leaves without stipules; in _Acer_
the leaves are palminerved, but imparipinnate in _Negundo_, a plant
frequently cultivated in gardens. The flowers are often unisexual,
polygamous (some species have ☿-, ♂-and ♀-flowers); sepals 5, petals
5 free, =stamens 8= (that is, 5 + 5, but the two median ones are
absent) inside a large disc. Fruit a samara (schizocarp) with 2
_winged, nut-like_ fruitlets (Fig. 474). In each of the 2 loculi of the
ovary are 2 ovules. Embryo _curved_, with thin, _folded_ cotyledons.
Endosperm absent.--~The inflorescences are racemes with a more or
less elongated main axis and terminal flower (which sometimes has 10
stamens); when the lateral branches are developed they are similar
to the main axis. In some species both corolla and petal-stamens are
suppressed. _Acer_ is pollinated by insects, _Negundo_ by the wind.--88
species; North Temperate zone. _Acer_ in the Tertiary from the
Oligocene. The following are native plants: Maple (_Acer campestre_),
Sycamore (_A. pseudoplatanus_, doubtful native). Important as avenue
trees and timber. Sugar is obtained from the spring sap of the Sugar
Maple (N. Am.).~
Order 5. =Malpighiaceæ.= A tropical (especially American) order
closely related to the Aceraceæ, having often the same form of
fruit (but 3-partite). Some species are lianes with anomalous
stem-structure. Leaves opposite. The flowers are regular or
obliquely zygomorphic (the plane of symmetry passing through
sepal 3), with S5, P5, A5 + 5, G3; 1 pendulous ovule in each
loculus. Important characteristics for identification are the
numerous grandular structures on the sepals. Peculiar 2-spined
hairs are found in some. _Malpighia_, _Bunchosia_, _Galphimia_,
_Tetrapterys_, _Heteropterys_, etc.--About 600 species.
Order 6. =Erythroxylaceæ.= Sepals 5, petals 5 (with a ligular
corona), 10 stamens in one bundle. Gynœceum 3-locular. Fruit
a drupe. Tropical (especially American) trees and shrubs, the
_Coca-plant_ (_Erythroxylon coca_) being best known. Its leaves
are considered by the inhabitants of Chile and Peru to be one
of the indispensable necessaries of life; they are chewed, and
possess intoxicating, exhilarating properties, and contain
the alkaloid cocaine, which is frequently employed as a local
anæsthetic.--103 species; chiefly in America.
Order 7. =Vochysiaceæ.= Trees; Trop. Am. 1 stamen.--140 species.
Order 8. =Trigoniaceæ.= Shrubs; Trop. Am.--30 species.
Order 9. =Tremandraceæ.= Polygalaceæ with regular flowers.--27
species. Australia.
[Illustration: FIG. 475.--Diagram of _Polygala_: _d_ a gland
in the posterior side of the flower; α and β the two caducous
bracteoles.]
Order 10. =Polygalaceæ.= Herbs or shrubs (some tropical species are
lianes) with scattered (rarely opposite), simple and most frequently
quite entire leaves, without stipules. The flowers are usually borne in
terminal spikes or racemes, and are strongly zygomorphic (_the plane
of symmetry being median_); they have 5 free sepals, the 2 _lateral
ones_ of which (4 and 5 in Figs. 475, 476) are very large, _petaloid_,
and frequently project on each side like the “wings” of a Pea-flower;
petals 5, of which the two lateral ones are wanting or rudimentary
(dotted on Fig. 475), and the _anterior_ “the _keel_” (Fig. 476 _c_)
is large, hollow and boat-shaped, and frequently with a lobed or
fimbriated edge (Fig. 476 _A_ and _B_, _c_); stamens 8, the two median
ones being absent, all _united_ into a tube split along the back, which
is also slightly united to the keel (the anthers, often 2 locular,
_open by pores_, Fig. 476 _B_, _st_); the 2 median carpels form a
bilocular ovary. 1 pendulous ovule in each loculus (Figs. 476 _C_,
475); capsule compressed with loculicidal dehiscence, rarely a nut.
_Polygala_ (Milk-wort).
470 species; distributed over the whole globe (none Arctic).
OFFICINAL: the root of _P. senega_, from N. Am. Some are used as
ornamental plants.
POLLINATION. The flowers of _Polygala_ are pollinated by insects
(chiefly bees). The fimbriated processes of the anterior petal
support the insect when it alights. The anthers lie on each
side of the stigma in the pouch of the anterior petal; the apex
of the style is spoon-shaped, and immediately behind it is a
viscid stigmatic lobe. In reaching the honey the proboscis of
the insect must come in contact with the pollen and the viscid
stigma, by which it is rendered sticky; this ensures the pollen
adhering to the proboscis and so being carried to other flowers.
[Illustration: FIG. 476.--_Polygala amara._ Parts of the flower
(mag.) _A_ Flower from side, 1-5 sepals: _c_ keel; _B_ flower
from above spread out: _st_ the 8 stamens; _c_ fimbriated edge of
“keel”; _C_ ovary with style and stigma.]