elobiae_. _A_, inflorescence of
arrowhead (_Sagittaria_), with a single female flower, x 1/2
(_Alismaceae_). _B_, section through the gynoecium, showing the numerous
single carpels, x 3. _C_, a ripe fruit, x 3. _D_, a male flower, x 1.
_E_, a single stamen, x 3. _F_, a leaf of _Sagittaria variabilis_,
x 1/6. _G_, ditch-moss (_Elodea_), with a female flower (_fl._), x 1/2.
(_Hydrocharideae_). _H_, the flower, x 2. _an._ the rudimentary
stamens. _st._ the stigma. _I_, cross-section of the ovary, x 4. _J_,
male inflorescence of eel-grass (_Vallisneria_), x 1. _K_, a single
expanded male flower, x 12. _st._ the stamen. _L_, a female flower,
x 1. _gy._ the stigma.]

The last family is the _Hydrocharideae_. They are submersed aquatics,
or a few of them with long-stalked, floating leaves. Two forms, the
ditch-moss (_Elodea_) (Fig. 91, _G_, _I_) and eel-grass
(_Vallisneria_) are very common in stagnant or slow-running water. In
both of these the plants are completely submersed, but there is a
special arrangement for bringing the flowers to the surface of the
water. Like the arrowhead, the flowers are unisexual, but borne on
different plants. The female flowers (_H_, _L_) are comparatively
large, especially in _Vallisneria_, and are borne on long stalks, by
means of which they reach the surface of the water, where they expand
and are ready for pollination. The male flowers (Fig. 91, _J_, _K_)
are extremely small and borne, many together, surrounded by a
membranous envelope, the whole inflorescence attached by a short
stalk. When the flowers are ready to open, they break away from their
attachment, and the envelope opens, allowing them to escape, and they
immediately rise to the surface where they expand and collect in great
numbers about the open female flowers. Sometimes these are so abundant
during the flowering period (late in summer) that the surface of the
water looks as if flour had been scattered over it. After pollination
is effected, the stem of the female flower coils up like a spring,
drawing the flower beneath the water where the fruit ripens.

The cells of these plants show very beautifully the circulation of the
protoplasm, the movement being very marked and continuing for a long
time under the microscope. To see this the whole leaf of _Elodea_, or
a section of that of _Vallisneria_, may be used.

CHAPTER XVII.

DICOTYLEDONS.

The second sub-class of the angiosperms, the dicotyledons, receive
their n