The spore-bearing generation may throughout its phylogenetic history have been independent at one part of its life, and have been derived by modification of individuals homologous with those of the sexual generation, and not by the progressive sterilization of a structure the whole of which was originally devoted to asexual reproduction.
Ferns, horse-tails, club mosses, &c., and Phanerogams or Flowering Plants) the main plant-body, that which we speak of in ordinary language as the plant, is called the sporophyte because it bears the asexual reproductive cells or spores.
Thus in Lemaneaceae asexual spores are unknown; in Batracho-spermum, Bonnemaisonia and Polysiphonia byssoides both kinds of sexual cells appear on the same plant; and in some cases the asexual cells may occur in conjunction with either the male or female sexual cells.
The Ascomycetes, at least the Carpoascomycetes, exhibit a well-marked alternation of sexual and asexual generations.
Here the asexual cells are borne upon the so-called Aglaozonia reptans and the sexual cells upon the plants known as Cutleria.
They belong to the group of Protozoa, and, as already explained, have a double cycle of existence: (I) a sexual cycle in the body of the mosquito, (2) an asexual cycle in the blood of human beings.
Sexual reproduction by oogonia and antheridia; asexual reproduction by zoospores or conidia.
They are characterized especially by the zygospores, but the asexual organs (sporangia) exhibit interesting series of changes, beginning with the typical sporangium of Mucor containing numerous endospores, passing to cases where, as in Thamnidium, these are accompanied with more numerous small sporangia (sporangioles) containing few spores, and thence to Chaetocladium and Piptocephalis, where the sporangioles form but one spore and fall and germinate as a whole; that is to say, the monosporous sporangium has become a conidium, and Brefeld regarded these and similar series of changes as explaining the relation of ascus to conidium in higher fungi.
True reproduction of the asexual kind occurs, however, in the formation of sporangia, particularly in the Chamaesiphonaceae.
Zoospores are of two kinds: (I) Those which come to rest and germinate to form a new plant; these are asexual and are zoospores proper.
Many Euchlorophyceae are endowed with both asexual and sexual reproduction.
Such are Coleochaete, Oedogonium, Cylindrocapsa, Ulothrix, Vaucheria, Volvox, &c. In others only the asexual method is yet known.
It will be remembered that in M usci, the asexual spore somewhat similarly gives rise to a protonema, from which the adult plant is produced as a lateral bud.
Both asexual and sexual reproduction occur among Euphaeophyceae.
The asexual cells are immotile spores arising in fours in sporangia from superficial cells of the thallus.
Among the Polychaeta the sexual worm is often more marked from the asexual form, so much so that these latter have been placed in different species or even genera.
It has been men, tioned that in the Nereids a sexual form occurs which differs structurally from the asexual worms, and was originally placed in a separate genus, Heteronereis; hence the name "Heteronereid" for the sexual worm.
On this view, therefore, at least two asexual generations (embryo and scolex) alternate with a sexual one (proglottides); and in the case of Staphylocystis the cyst contains two asexually produced generations, so that in such forms three stages (embryo, primary scolex-buds, secondary scolices) intervene between the proglottis of a Cestode and that of its offspring.
The scheme of Brefeld, which was based on the view that the Ascomycetes and Basidiomycetes were completely asexual and that these two groups had been derived from one division (Zygomycetes) of the Phycomycetes, has been very widely accepted.
The root is an axis which never bears either leaves or the proper reproductive organs (whether sexual or asexual) of the plant.
Asexual reproduction only in Naids.
No asexual generation.
The green (or blue-green) cells were termed gonidia by Wallroth, who looked upon them as asexual reproductive cells, but when it was later realized that they were not reproductive elements they were considered as mere outgrowths of the hyphae of the thallus which had developed chlorophyll.
The cycle of development taken by the Malacocotylea has been generally regarded as an alternation of one or more asexual generations with a sexual one.