Order 3. =Callitrichaceæ.= Aquatic plants, growing at the bottom
of shallow water, with opposite, simple, undivided, entire,
exstipulate leaves, which are generally crowded and form a
rosette in the apex of the branches. The flowers are unisexual
(monœcious) and borne singly in the leaf-axils; they have no
perianth, but are provided with two delicate bracteoles; the
♂-flowers consist of only _1 terminal stamen_ (Fig. 462);
the ♀-flowers of a bicarpellate gynœceum (Fig. 463) which is
originally 2-locular, but later on becomes 4-locular, as in the
case of the gynœceum of the Labiatæ, by the formation of a false
partition-wall; in each loculus there is 1 pendulous ovule with
the micropyle turned outwards. Fruit a _4-partite schizocarp_
(Fig. 464). 25 species.--_Callitriche._
Order 4 (?). =Empetraceæ.= 4 species. _Empetrum_; _E. nigrum_
(Crowberry) is a heather-like, moorland, evergreen undershrub
with linear leaves, having a deep groove closed with hairs, on
the under side. The _erect ovules_ show the greatest deviation
from the Euphorbiaceæ. Diœcious (and ☿); S3, P3; in the
♂-flower, 3 stamens; in the ♀-flower, a 6–9-locular ovary. Fruit
a _drupe_.