The algal cells are usually controlled in their growth by the hyphae and are prevented from forming zoospores, and in some cases, as already described, the algal cells are killed sooner or later by the fungus.
In 1865 De Bary suggested the possibility that such lichens as Collema, Ephebe, &c., arose as a result of the attack of parasitic Ascomycetes upon the algae, Nostoc, Chroococcus, &c. In 1867 the observations of Famintzin and Baranetzky showed that the gonidia, in certain cases, were able to live outside the lichen-thallus, and in the case of Physcia, Evernia and Cladonia were able to form zoospores.
c, Conidium emitting zooC. Formation of zoospores by spores.
z, Free zoospores.
In Peronospora, Saprolegnia, &c., the ends of the branches swell up into sporangia, which develop zoospores in their interior (zoosporangia), or their contents become oospheres, which may be fertilized by the contents of other branches (antheridia) and so form egg-cases (oogonia).
Sexual reproduction by oogonia and antheridia; asexual reproduction by zoospores or conidia.
Mycelium poorly developed or absent; oogonia and antheridia (without antherozoids) known in some cases; zoospores common: Chytridiaceae.
When the free ends of the hyphae emerge again into the air they swell up into spherical bodies which may either fall off and behave as conidia, each putting out a germ-tube and infecting the host; or the germ-tube itself swells up into a zoosporangium which develops a number of zoospores.
Each "conidium" contains numerous nuclei and is really a zoosporangium, as after dispersal it breaks up into a number of zoospores.
tophthora infestans passing K, Germination of the zoospores through the stomata D, on formed in the sporangia.
Motile zoospores which escape from the zoosporangium are present except in Aplanes.
They are usually included in Oomycetes, but their simple structure, minute size, usually uniciliate zoospores, and their negative characters would justify their retention as a separate group. It contains less than 200 species, chiefly parasitic on or in algae and other water-plants or animals, of various kinds, or in other fungi, seedlings, pollen and higher plants.
On the other hand, the uniciliate zoospores of Polyphagus have slightly amoeboid movements, and in this and the pseudopodium-like nature of the protoplasmic processes, such forms suggest resemblances to the Myxomycetes.
A certain inequality in the character of the two cilia of the zoospores of some of the members of the group has earned for it the title Heterokontae, from the Greek xovros, a punting-pole.
With regard to true reproduction, which is characterized by the formation of special cells, the group Euchlorophyceae is characterized by the production of zoospores (Gr.
In all other cases, zoospores are uninucleate bodies.
Zoospores arise in cells of ordinary size and form termed zoosporangia.
In coenocytic forms the zoospores would seem to arise simultaneously, probably because many nuclei are already present.
The escape of zoospores is effected by the degeneration of the sporangial wall (Chaetophora), or by a pore (Cladophora), a slit (Pediastrum), or a circular fracture (Oedogonium).
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.
When zoospores come to rest, a new cell is formed and germination ensues at once.
This is in consonance with the facts already mentioned that zoospores germinate forthwith, and that the sexually-produced cell or zygote enters upon a period of rest.
The germination of a zygospore or oospore is effected by the rupture of an outer cuticularized exosporium; then the cell may protrude an inner wall, the endosporium, and grow out into the new plant (Vaucheria), or the contents may break up into a first brood of zoospores.
Aplanospores would seem to represent zoospores arrested in their development; without reaching the stage of motility, they germinate within the sporangium.
Both aplanospores and akinetes may germinate with or without the formation of zoospores at the initial stage.
C. Ulothrix sp., zoospores escapG.
They are unilocular, each producing a small number of zoospores.
Bornet, however, called attention in 1871 to the fact that two kinds of plurilocular sporangia occurred in certain species of the genus Ectocarpus - somewhat transparent organs of an orange tint producing small zoospores, and also more opaque organs of a darker colour producing relatively larger zoospores.
In these cases, however, the potential gametes may, failing conjugation, germinate directly, like the zoospores derived from unilocular sporangia.
The assertion of Areschoug that conjugation occurs among zoospores derived from unilocular sporangia, in the case of Dictyosiphon hippuroides, is no doubt to be ascribed to error of observation.
Multiplication takes place in some cases by the endogenous formation of zoospores, the organism having come to rest; in others by longitudinal division, when the organism is still motile.
Both plants multiply solely by means of zoospores.
It is noteworthy that although all the members of the group are aquatic no zoospores are produced, a negative character common to them and the Blue-green Algae.
Among Chlorophyceae it is often the case that the oospore on germination divides up directly to form a brood of zoospores.
In Sphaeroplea it is only at this stage that zoospores are formed at all; but in most cases, such as Oedogonium, Ulothrix, Coleochaete, similar zoospores are produced again and again upon the thallus, and the product of the oospore may be regarded as merely a first brood of a series.
But it is difficult to apply such a term at all to those cases in which there intervene between the oospore and the next sexual stage a series of generations, the zoospores of which are all precisely similar.
He has also seen Pleurococcus viridis dividing so as to form a filament, but has not succeeded in seeing the formation of zoospores as described by Chodat.
One is shown more highly magnified (400 diameters) at F; the contained protoplasm breaks up into a definite number of parts as at G, forming eight minute mobile bodies called "zoospores," each zoospore being furnished with two extremely attentuated vibrating hairs termed "cilia," as shown at H.
These zoospores escape and swim about in any film of moisture, and on going to rest take a spherical form, germinate and produce threads of mycelium as at K.
Such a behaviour is very similar to the production of zoospores which is so common in many filamentous algae.
i) of the same nature as those of the zoospores and antherozoids of algae, mosses, &c.
The affinities of the Dinoflagellata are certainly with those Cryptomonadine Flagellates which possess two unequal flagella; the zoospores or young of the Cystoflagellates are practically colourless Dinoflagellates.
biflagellate zoospores are cleaved within this vesicle which then breaks down to release the zoospores. [© Jim Deacon] Fig.
motile zoospores that are readily carried in soil water and thus spread the disease within the crop.
Oospores germinate in wet soils to produce a sporangium which then rapidly differentiates to release zoospores.
sporangium a hyaline mycelium bearing lemon-shaped sporangia which released motile zoospores after chilling in water, consistent with P. infestans.
At maturity, the protoplasts convert to sporangia, which release zoospores into the soil.
Oospores germinate to release motile zoospores that are readily carried in soil water and thus spread the disease within the crop.
Eventually, the resting spores germinate to release haploid zoospores which repeat the infection cycle. [© Jim Deacon] Fig.
See Chapter 10 for details. [© Jim Deacon] Fig 10.20 Accumulation and encystment of Pythium zoospores on wheat roots.
zoospores, conidia, endogonidia, stylospores, &c.; (2) paulospores, due to simple rearrangement of cell-contents, and subserving the persistence of the fungus through periods of exigency, e.g.
Passing over the locomotor activity of zoospores (Pythium, Peronospora, Saprolegnia) we often find spores held under tension in sporangia (Pilobolus) or in asci (Peziza) until ripe, and then forcibly shot out by the sudden rupture of the sporangial wall under the pressure of liquid behind - mechanism comparable to that of a pop-gun, if we suppose air replaced by watery sap. Even a single conidium, held tense to the last moment by the elastic cell-wall, may be thus shot forward by a spurt of liquid under pressure in the hypha abstricting it (e.g.
The other genera are more purely parasitic; the mycelium usually sends haustoria into the cells of the host and puts out branched, aerial conidiophores through the stomata, the branches of which abstrict numerous "conidia"; these either germinate directly or their contents break up into zoospores (fig.
Peronospora; while others emit zoospores - e.g.
The mycelium from the germinating sporangia or zoospores soon finds its way into the tissues of the potato leaf by the organs of transpiration, and the process of growth already described is repeated -over and over again till the entire potato leaf, or indeed the whole plant, is reduced to putridity.
This had a hyaline mycelium bearing lemon-shaped sporangia which released motile zoospores after chilling in water, consistent with P. infestans.
The word usage examples above have been gathered from various sources to reflect current and historial usage. They do not represent the opinions of YourDictionary.com.