In this state of my knowledge the subject was taken up in 1818, by my lamented friend the late Mr. Thomas Smith, who, eminently qualified for an investigation where minute accuracy and great experience in microscopical observation were necessary, succeeded in ascertaining the very general existence of the foramen in the membranes of the Ovulum. But as the foramina in these membranes invariably correspond both with each other and with the apex of the nucleus, a test of the direction of the future Embryo was consequently found nearly as universal, and more obvious than that which I had previously employed.
To determine in what degree this account of the vegetable Ovulum differs from those hitherto given, and in some measure, that its correctness may be judged of, I shall proceed to state the various observations that have been actually made, and the opinions that have been formed on the subject, as briefly as I am able, taking them in chronological order.
In 1672, Grew* describes in the outer coat of the seeds of many Leguminous plants a small foramen, placed opposite to the radicle of the Embryo, which, he adds, is "not a hole casually made, or by the breaking off of the stalk," but formed for purposes afterwards stated to be the aeration of the Embryo, and facilitating the passage of its radicle in germination. It appears that he did not consider this foramen in the testa as always present, the functions which he ascribes to it being performed in cases where it is not found, either, according to him, by the hilum itself, or in hard fruits, by an aperture in the stone or shell.
(*Footnote. Anatomy of Veget. begun page 3. Anatomy of Plants page 2.)
In another part of his work* he describes and figures, in the early state of the Ovulum, two coats, of which the outer is the testa; the other, his middle membrane, is evidently what I have termed nucleus, whose origin in the Ovulum of the Apricot he has distinctly represented and described.
(*Footnote. Anatomy of Plants page 210 table 80.)
Malpighi, in 1675,* gives the same account of the early state of the Ovulum; his secundinae externae being the testa, and his chorion the nucleus. He has not, however, distinguished, though he appears to have seen, the foramen of Grew, from the fenestra and fenestella, and these, to which he assigns the same functions, are merely his terms for the hilum.
(*Footnote. Anatome Plant. page 75 et 80.)
In 1694, Camerarius, in his admirable essay on the sexes of plants,* proposes, as queries merely, various modes in which either the entire grains of pollen, or their particles after bursting, may be supposed to reach and act upon the unimpregnated Ovula, which he had himself carefully observed. With his usual candour, however, he acknowledges his obligation on this subject to Malpighi, to whose more detailed account of them he refers.
(*Footnote. Rudolphi Jacobi Camerarii de sexu plantarum epistola page 8 46 et seq.)