The nucellus is the only macrosporangium which never opens; _the macrospore remains enclosed in it_, and _the macrosporangium remains attached to the mother-plant_. It is therefore essential that the _method of fertilisation_ which is employed should be very different from that of the Cryptogams. _The pollen-grains must be transferred to the ovule_, and retained either by a drop of mucilage at the micropyle (Gymnosperms) or by the stigma on the carpels (Angiosperms). Fertilisation by spermatozoids, which are freely motile in water, is abandoned in the Phanerogams. Many other modifications, unknown in plants of more simple structure, take place, for instance, in the shoots which bear the fertile leaves; especially in the form of the stem or _thalamus_ (hypogynous, perigynous, epigynous); in the development of the perianth which stands in intimate connection with the special means employed to effect fertilisation; with respect to the different grades of union found in the leaves; in the union of the flowers into aggregations of a higher order (inflorescences), and at the same time the production of “floral-leaves” (page 235). =The sexual generation. The Fertilisation.= The sexual generation in the _Mosses_ is relatively well developed, because not only the protonema, but all the other vegetative parts of the Moss-plant, in addition to the archegonia and antheridia, belong to it. In the groups which follow, a gradual but increasing reduction of the sexual generation takes place, and at the same time an indication of sex is found in the prothallia, which finds expression in the forms of the spores themselves. In the majority of cases among the _isosporous_ Vascular Cryptogams, the sexual generation--prothallium--is a green, leafy expansion which can sustain itself by the assimilation of carbonic acid, and by the absorption of nutriment from the soil by means of root-hairs. In some plants (_Ophioglossaceæ_, _Lycopodium annotinum_) the prothallium is a subterranean, pale, tubercular body, but in these instances it is relatively large. In the _heterosporous_ Vascular Cryptogams and in the _Phanerogams_, the prothallium is much more reduced, both as regards its size, and also with respect to the number and structure of the antheridia and archegonia.