ements for cross fertilization common
to hermaphrodite flowers in various different families of plants, which
have received special names, _Dichogamy_ and _Heterogony_.

339. =Dichogamy= is the commoner case. Flowers are _dichogamous_ when
the anthers discharge their pollen either before or after the stigmas of
that flower are in a condition to receive it. Such flowers are

_Proterandrous_, when the anthers are earlier than the stigmas, as in
Gentians, Campanula, Epilobium, etc.

_Proterogynous_, when the stigmas are mature and moistened for the
reception of pollen, before the anthers of that blossom are ready to
supply it, and are withered before that pollen can be supplied.
Plantains or Ribworts (mostly wind-fertilized) are strikingly
proterogynous: so is Amorpha, our Papaws, Scrophularia, and in a less
degree the blossom of Pears, Hawthorns, and Horse-chestnut.

340. In Sabbatia, the large-flowered species of Epilobium, and
strikingly in Clerodendron, the dichogamy is supplemented and perfected
by movements of the stamens and style, one or both, adjusted to make
sure of cross fertilization.

341. =Heterogony.= This is the case in which hermaphrodite and fertile
flowers of two sorts are produced on different individuals of the same
species; one sort having higher anthers and lower stigmas, the other
having higher stigmas and lower anthers. Thus reciprocally disposed, a
visiting insect carries pollen from the high anthers of the one to the
high stigma of the other, and from the low anthers of the one to the low
stigma of the other. These plants are practically as if dioecious,
with the advantage that both kinds are fruitful. Houstonia and
Mitchella, or Partridge-berry, are excellent and familiar examples.
These are cases of

_Heterogone Dimorphism_, the relative lengths being only short and long
reciprocally.

_Heterogone Trimorphism_, in which there is a mid-length as well as a
long and a short set of stamens and style; occurs in Lythrum Salicaria
and some species of Oxalis.

342. There must be some essential advantage in cross fertilization or
cross breeding. Otherwise all these various, elaborate, and exquisitely
adjusted adaptations would be aimless. Doubtless the advantage is the
same as that which is realized in all the higher animals by the
distinction of sexes.

Sec. 2. ACTION OF POLLEN, AND FORMATION OF THE EMBRYO.

343. =Pollen-growth.= A grain of pollen may be justly likened to one of
the simp