BOTANY.]
NATURAL
HISTORY.
23
to comprehend in what manner this influence can reach
its apex externally, except on the supposition, not hastily
to be admitted, of an impregnating aura filling the cavity
of the ovarium; or by the complete separation of the fecundating tubes from the placentee, which, however, in such
cases I have never been able to detect.
It would entirely remove the doubts that may exist respecting the point of impregnation, if cases could be produced where the ovarium was either altogether wanting, or
so imperfectly formed, that the ovulum itself became directly exposed to the action of the pollen, or its fovilla; its
apex, as well as the orifice of its immediate covering, being
modified and developed to adapt them to this economy.
But such, I believe, is the real explanation of the structure of Cycadese, of Coniferse, of Ephedra, and even of
Gnetum, of which Thoa of Aublet is a species.
To this view the most formidable objection would be
removed, were it admitted, in conformity with the preceding
observations, that the apex of the nucleus, or supposed
point of impregnation, has no organic connexion with the
parietes of the ovarium. In support of it, also, as far as regards the direct action of the pollen on the ovulum, numerous instances of analogous economy in the animal kingdom may be adduced.
The similarity of the female flower in Cycadeae and Coniferse to the ovulum of other pheenogamous plants, as I have
described it, is indeed sufficiently obvious to render the
opinion here advanced not altogether improbable. But the
Helianthemum, Hudsonia and Lechea), I communicated to Dr.
Hooker, by whom it is noticed in his Flora Scotica, (p. 2S4,) published in 1821; where, however, an observation is added respecting
Gfertner's description of Cistus and Helianthemum, for which I am
not accountable.