By 1830 the mechanism of fertilisation came to light in Amici's discovery of the pollen tube which he traced from the stigma to the micropyle. The microscopic aspect of the problem was taken up with great energy by Schleiden and brought to the forefront as the burning question of the early forties. The theory of Schleiden, which applied in particular to the flowering plants, made its influence felt to such an extent in the search for evidence of sexuality among the Cryptogams, that we may conveniently state in a few words in what it consisted. Schleiden traced the pollen tube into the micropyle, and thence to the nucellus where it depressed or invaginated the apex of the embryo-sac, and in the recess or indentation so produced the tip of the pollen tube was converted into the embryo--its actual apex being represented by the plumule. This theory was the lineal descendant in modernised trappings of the old view expressed by Morland and others at the beginning of the eighteenth century that the embryo was contained in the pollen grain, and that the ovule was no more than the brood chamber whither it must be brought to undergo further development. This erroneous interpretation of the true facts was always repudiated by Amici, and was finally overthrown by Hofmeister and Radlkofer in the early fifties. In this connection we may note in passing Henfrey's careful paper on the impregnation of _Orchis Morio_, published in 1856, which fully corroborated Amici. In this paper the relations of pollen tube, embryo-sac, egg-cell, suspensor and embryo were correctly interpreted, and the new point established, contrary to the assertions of previous observers, that the ovum or "germinal-vesicle," prior to fertilisation, was a naked, unwalled cell. _Sexuality in Cryptogams._ By far the most important question that came to a head in Henfrey's time was that of the morphological relationships of the Cryptogams and flowering plants. Hitherto these had remained altogether obscure in the absence of reliable data based on the proper application of the microscope to the elucidation of the life histories of the lower plants. Under the influence of the Linnaean school, which had taken deep root in this country, as elsewhere, the systematic study of flowering plants had been widely pursued, and in so far as the ferns were concerned their homologies were commonly interpreted in terms of the flowering plants. Without any real guidance in fact, a great diversity of views of these homologies found expression. The following, taken from Lindley, may serve to illustrate their general nature. The sorus was regarded as a sort of compound fruit, the sporangium as a carpel, the annulus as its midrib, and the spores as the seeds. Speculations such as these are of the same order as the crude conjectures which with less excuse relieve the answer books of examination candidates at the present time.