ation to the
stigma, were necessary for the production of perfect seed, but the
stages of the process of fertilization remained unexplored. The matter
seemed involved in mystery, and no one attempted to raise the veil which
hung over the subject of embryogeny. The general view was, that the
embryo originated in the ovule, which was in some obscure manner
fertilized by the pollen.

In 1815 L.C. Treviranus, professor of botany in Bonn, roused the
attention of botanists to the development of the embryo, but although he
made valuable researches, he did not add much in the way of new
information. In 1823 G.B. Amici discovered the existence of pollen
tubes, and he was followed by A.T. Brongniart and R. Brown. The latter
traced the tubes as far as the nucleus of the ovule. These important
discoveries mark a new epoch in embryology, and may be said to be the
foundation of the views now entertained, which were materially aided by
the subsequent elucidation of the process of cytogenesis, or
cell-development, by Schleiden, Schwann, Mohl and others. The whole
subject of fertilization and development of the embryo has been more
recently investigated with great assiduity and zeal, as regards both
cryptogamous and phanerogamous plants, and details must be sought in the
various special articles. The observations of Darwin as to the
fertilization of orchids, _Primula, Linum_ and _Lythrum_, and other
plants, and the part which insects take in this function, gave an
explanation of the observations of Christian Konrad Sprengel, made at
the close of the 18th century, and opened up a new phase in the study of
botany, which has been followed by Hermann Muller, Federico Delpino and
others, and more recently by Paul Knuth.

One of the earliest workers at plant physiology was Stephen Hales. In
his _Statical Essays_ (1727) he gave an account of numerous experiments
and observations which he had made on the nutrition of plants and the
movement of sap in them. He showed that the gaseous constituents of the
air contribute largely to the nourishment of plants, and that the leaves
are the organs which elaborate the food; the importance of leaves in
nutrition had been previously pointed out by Malpighi in a short account
of nutrition which forms an appendix to his anatomical work. The birth
of modern chemistry in the work of J. Priestley and Lavoisier, at the
close of the 18th century, made possible the scientific study of
plant-nutrition, though Jan