Two great divisions are recognized in the Fungi: (i.) the Pycomycetes or Algal Fungi, which retain a definitely sexual method of reproduction as well as asexual (vegetative) methods, and (ii.) the Mycomycetes, characterized by extremely reduced or very doubtful sexual reproduction.
With certain rare exceptions the Saccharomycetes have three methods of asexual reproduction: I.
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.
The root is an axis which never bears either leaves or the proper reproductive organs (whether sexual or asexual) of the plant.
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.
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.
Asexual reproduction universal.
Asexual reproduction only in Naids.
No asexual generation.
Reproduction is mainly asexual, the females producing living young without the agency of a male.
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.
Von Linstow has indeed suggested that Gyrodactylus is a larval form capable of reproduction by an asexual method.
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.
He divided plants into sexual and asexual, the former being Phanerogamous or flowering, and the latter Cryptogamous or flowerless.
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.
In one view they are mere asexual conidia, and the term pycnoconidia is accordingly applied since they are borne in structures like the non-sexual pycnidia of other fungi.
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.
Sexual reproduction by oogonia and antheridia; asexual reproduction by zoospores or conidia.
Mycelium well developed; sexual reproduction by zygospores; asexual reproduction by sporangia and conidia.
Sexual reproduction as above, asexual by sporangia or conidia or both: Mucoraceae.
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.
The Ascomycetes, at least the Carpoascomycetes, exhibit a well-marked alternation of sexual and asexual generations.
Asexual reproductive cells are not infrequent, but sexual reproduction even in its initial stages is unknown.
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.
From a comparison of those Euchlorophyceae which have been most closely investigated, it appears probable that sexual reproductive cells have in the course of evolution arisen as the result of specialization among asexual reproductive cells, and that in turn oogamous reproduction has arisen as the result of differentiation of the two conjugating cells into the smaller male gamete and the larger male gamete.
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.
When a species resorts to both methods, it is generally found that the asexual method prevails in the early part of the vegetative period and the sexual towards the close of that period.
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.
Fucaceae are marked by an entire absence of the asexual method.
The asexual cells are immotile spores arising in fours in sporangia from superficial cells of the thallus.
The discovery by Brebner of the specific identity of Haplospora globosa and Scaphospora speciosa marks an important step in the advance of our knowledge of the group. Three kinds of reproductive organs are known: first, sporangia, which each give rise to a single tetra-, or multi-nucleate non-motile, probably asexual spore; second, plurilocular sporangia, which are probably antheridia, generating antherozoids; and third, sporangia, which are probably oogonia, giving rise to single uninucleate non-motile oospheres.
The asexual organs in the case of Cutleria multifida arise on a crustaceous form, Aglaozonia reptans, formerly considered to be a distinct species.
It would thus seem that the explanation of the existence of two kinds of sporangia, unilocular and plurilocular, among Phaeosporeae, lies in the fact that unilocular sporangia are for asexual reproduction, and that plurilocular sporangia are gametangia - potential or actual.
Reproduction is both asexual and sexual.
As a rule the asexual cells, and the male and female sexual cells arise upon different plants, so that the species may be said to be trioecious.
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 asexual cells are termed tetraspores on account of the usual occurrence of four in each sporangium.
Sexual and asexual reproduction prevail.
While the spore of Bryophyta on germination gives rise to the sexual plant, the carpospore of the alga may give rise on germination to a plant bearing a second sort of asexual cells, viz.
Here the asexual cells are borne upon the so-called Aglaozonia reptans and the sexual cells upon the plants known as Cutleria.
It will be remembered that, as in most Florideae, the male, female and asexual plants are distinct in this genus.
As the result of fertilization of an ovum produced by this, the fern plant (sporophyte, asexual generation) originates; from it spores are ultimately set free,.
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.