in the female organ and to excite it to further development
(1849, 1851). Soon after the sexual act was observed in
various Algae, and these afforded the best opportunity for
solving by the aid of the microscope the questions which
experiment had still left open. Thuret showed in 1854, how
the large egg-cells in species of Fucus are surrounded and fertilised by spermatozoids, and he even succeeded in producing hybrids by fertilising the egg-cells of one species
with the spermatozoids of another; but it was still uncertain
whether simple contact of the male and female organs was
sufficient, or whether fertilisation is due to the mingling of the substance of the spermatozoid and the germ-cell; the question was settled by Pringsheim in 1855; he saw the male organ of fertilisation of a fresh-water alga penetrate into
the substance of the egg-cell and be dissolved in it, and
this proceeding was afterwards observed in higher Cryptogams
and is represented in its simplest form in the sexual act of
the Conjugatae, which De Bary described at length in 1858 and like Vaucher regarded as a sexual process.
When we consider to what an extent the time and power of work of the most eminent botanists was devoted after 1840 to long and difficult observations on the minute anatomy of plants, on cell-formation, embryology and the history of the development of organs, we cannot wonder if other parts of vegetable physiology, which require experiments on vegetation in plants, were cultivated but little and by the way only; but these studies also gained firmer footing in the advance of phytotomy, which supplied the physiologist with a more definite idea of the organism in which the phenonema of vegetative life are produced.
The chemistry of the food of plants was one of the strictly physiological subjects, which like the sexual theory was studied without intermission and with considerable success in the period from 1840 to 1860, but chiefly or entirely by chemists, who connected their investigations into the processes of
