Phylum Thallophyta - The Algae - Phycomycetes - Saprolegniales
The Saprolegniales are Phycomycetes in which the vegetative mycelium is profusely branched and coenocytic, septa occurring rarely, in older branches and at the separation of the reproductive organs. Asexual reproduction is by zoospores, which exhibit a peculiar condition of development. In the simpler genera the zoospores when liberated are pyriform and have two equal apical flagella. They then rest and encyst, emerging again with equal lateral flagella and reniform in shape. In the more advanced members the earlier phase of this condition, which is termed diplanetism, may be missing. Sexual reproduction is oogamous, there being more than one oosphere in each oogonium. The antheridium is a filiform structure and fertilization is by a fertilization tube.
The species may be monoecious or dioecious; almost all are saprophytes and are characteristic inhabitants of soils. Many are also found in fresh water.
Consider in detail one example, Saprolegnia.
Saprolegnia
The separation of the species and even the genera of the family Saprolegniaceae is by no means easy. The genera are separated mainly by the method of emergence of the asexual zoospores, the species by minor details of size in the reproductive organs.
In nature these Fungi grow chiefly on decaying plant remains lying at the bottom of ponds and slow-flowing streams, or on humus in soils. Recent work seems to indicate that while some species of Saprolegnia occur regularly in water, others are more typically soil-inhabiting organisms living on humus. Many of these species occur in water only as a result of their reproductive bodies being washed out of the soil by the rain; though if circumstances are favourable they will continue to develop under aquatic conditions. The name" Water Moulds," which is collectively given to this group, is therefore somewhat of a misnomer. They are generally obtained in the laboratory by placing dead flies or the cut seeds of hemp or cress in pond water, when, after a few days, a copious growth of the Fungus will appear on the bait. The Fungus produces a transparent whitish film over the substratum, which consists of a large number of very fine hyphae, collectively spoken of as the mycelium. These hyphae are very long and are not divided by septa into separate cells, hence they must be regarded as coenocytic. Only the sex organs are cut off by septa. The hyphae may be branched or simple, and the degree of branching varies very much among different species, and according to the conditions under which the Fungus is growing. In fact, not only the appearance but the whole sequence of events which make up the life history of Saprolegnia can be considerably altered according to the environmental conditions.
Saprolegnia grows rapidly, and under favourable circumstances asexual reproduction occurs within a few days of the appearance of the Fungus on the substratum. After a few days of active asexual reproduction the sex organs appear, and this is followed by a falling off in the formation of asexual bodies.
ASEXUAL REPRODUCTION
The asexual reproductive organs are zoosporangia, which are produced terminally at the ends of the hyphae. They are long tubular structures, tapering at the tip. The contents become very dense as protoplasm migrates from the hypha towards the tip. Finally the structure is cut off at its base by a septum. Then the contents divide up into a large number of uninucleated zoospores, each of 'vvhich is pear-shaped and provided with a pair of apical flagella. When mature they escape one after another through an opening in the top of the sporangium. They swim away, but their activity is of brief duration, for after a short while they withdraw their flagella and become spherical. After a period of rest they again become active, only this time the cells are kidney-shaped and are provided with two lateral flagella. This phenomenon, consisting of two motile stages separated by a resting stage, is spoken of as diplanetism. The kidney-shaped zoospores, after a further period of activity, settle down and germinate into fresh hyphae. The second motile phase may be repeated more than once.
Although the behaviour in the genus Saprolegnia shows the true picture of diplanetism the conditions in allied genera are somewhat different, and since they are often encountered mixed with Saprolegnia it will be necessary to say something about them.
In the genus Achlya the zoospores, instead of being discharged from the zoosporangium in a series, like bullets from a machine gun, are ejected in a cluster, forming a large mass around the apex of the zoosporangium. Here they immediately encyst and lose their flagella, thus passing into the second
hase of a diplanetic condition. After a rest of several hours they emerge with reniform bodies and swim away with lateral flagella.
In the genus Dictyuchus, which may also be found in hemp-seed cultures, the zoospores are so closely packed in the zoosporangium that they assume a hexagonal pattern. In this genus they do not emerge through an apical pore but by numerous pores all over the surface of the sporangium. They are kidney-shaped with lateral flagella. Thus in this genus the encysted stage is passed through within the zoosporangium.
In the genus Thraustotheca the condition is somewhat similar, except that the spores are liberated by the breakdown of the wall of the sporangium in a non-motile spherical state and soon become reniform with lateral flagella.
Finally, in the genus Aplanes, a less common one, there is no motile stage at all, and the spores germinate within the zoosporangium by means of germ tubes which penetrate the wall of the sporangium and reach the exterior.
As will be seen in subsequent examples, there appears to be a progressive tendency among these groups of Phycomycetes to change from a condition with apically flagellate pyriform zoospores to one of reniform laterally flagellate zoospores. This is brought about by the interpolation of an encysted condition. In the more advanced types the production of the first type of zoospore disappears completely, and the second stage takes place within the sporangium, or in a vesicle formed as an outgrowth from it, so that the only motile stage is the third. Finally we see that this motility is lost and the spores germinate by a germ tube within the sporangium.
In Saprolegnia, after the liberation of the zoospores from the zoosporangium, secondary sporangia may be developed. This may be effected, either by the development of a second sporangium from the base within the primary one, or by the production of a hypha from the same point, which grows up through the opening in the tip of the primary sporangium to form a secondary sporangium, sometimes some \Yay above the first. This is termed proliferation. Several successive generations of zoosporangia may be formed in this way (Fig. 203).
The Saprolegniales are Phycomycetes in which the vegetative mycelium is profusely branched and coenocytic, septa occurring rarely, in older branches and at the separation of the reproductive organs. Asexual reproduction is by zoospores, which exhibit a peculiar condition of development. In the simpler genera the zoospores when liberated are pyriform and have two equal apical flagella. They then rest and encyst, emerging again with equal lateral flagella and reniform in shape. In the more advanced members the earlier phase of this condition, which is termed diplanetism, may be missing. Sexual reproduction is oogamous, there being more than one oosphere in each oogonium. The antheridium is a filiform structure and fertilization is by a fertilization tube.
The species may be monoecious or dioecious; almost all are saprophytes and are characteristic inhabitants of soils. Many are also found in fresh water.
Consider in detail one example, Saprolegnia.
Saprolegnia
The separation of the species and even the genera of the family Saprolegniaceae is by no means easy. The genera are separated mainly by the method of emergence of the asexual zoospores, the species by minor details of size in the reproductive organs.
In nature these Fungi grow chiefly on decaying plant remains lying at the bottom of ponds and slow-flowing streams, or on humus in soils. Recent work seems to indicate that while some species of Saprolegnia occur regularly in water, others are more typically soil-inhabiting organisms living on humus. Many of these species occur in water only as a result of their reproductive bodies being washed out of the soil by the rain; though if circumstances are favourable they will continue to develop under aquatic conditions. The name" Water Moulds," which is collectively given to this group, is therefore somewhat of a misnomer. They are generally obtained in the laboratory by placing dead flies or the cut seeds of hemp or cress in pond water, when, after a few days, a copious growth of the Fungus will appear on the bait. The Fungus produces a transparent whitish film over the substratum, which consists of a large number of very fine hyphae, collectively spoken of as the mycelium. These hyphae are very long and are not divided by septa into separate cells, hence they must be regarded as coenocytic. Only the sex organs are cut off by septa. The hyphae may be branched or simple, and the degree of branching varies very much among different species, and according to the conditions under which the Fungus is growing. In fact, not only the appearance but the whole sequence of events which make up the life history of Saprolegnia can be considerably altered according to the environmental conditions.
Saprolegnia grows rapidly, and under favourable circumstances asexual reproduction occurs within a few days of the appearance of the Fungus on the substratum. After a few days of active asexual reproduction the sex organs appear, and this is followed by a falling off in the formation of asexual bodies.
ASEXUAL REPRODUCTION
The asexual reproductive organs are zoosporangia, which are produced terminally at the ends of the hyphae. They are long tubular structures, tapering at the tip. The contents become very dense as protoplasm migrates from the hypha towards the tip. Finally the structure is cut off at its base by a septum. Then the contents divide up into a large number of uninucleated zoospores, each of 'vvhich is pear-shaped and provided with a pair of apical flagella. When mature they escape one after another through an opening in the top of the sporangium. They swim away, but their activity is of brief duration, for after a short while they withdraw their flagella and become spherical. After a period of rest they again become active, only this time the cells are kidney-shaped and are provided with two lateral flagella. This phenomenon, consisting of two motile stages separated by a resting stage, is spoken of as diplanetism. The kidney-shaped zoospores, after a further period of activity, settle down and germinate into fresh hyphae. The second motile phase may be repeated more than once.
Although the behaviour in the genus Saprolegnia shows the true picture of diplanetism the conditions in allied genera are somewhat different, and since they are often encountered mixed with Saprolegnia it will be necessary to say something about them.
In the genus Achlya the zoospores, instead of being discharged from the zoosporangium in a series, like bullets from a machine gun, are ejected in a cluster, forming a large mass around the apex of the zoosporangium. Here they immediately encyst and lose their flagella, thus passing into the second
hase of a diplanetic condition. After a rest of several hours they emerge with reniform bodies and swim away with lateral flagella.
In the genus Dictyuchus, which may also be found in hemp-seed cultures, the zoospores are so closely packed in the zoosporangium that they assume a hexagonal pattern. In this genus they do not emerge through an apical pore but by numerous pores all over the surface of the sporangium. They are kidney-shaped with lateral flagella. Thus in this genus the encysted stage is passed through within the zoosporangium.
In the genus Thraustotheca the condition is somewhat similar, except that the spores are liberated by the breakdown of the wall of the sporangium in a non-motile spherical state and soon become reniform with lateral flagella.
Finally, in the genus Aplanes, a less common one, there is no motile stage at all, and the spores germinate within the zoosporangium by means of germ tubes which penetrate the wall of the sporangium and reach the exterior.
As will be seen in subsequent examples, there appears to be a progressive tendency among these groups of Phycomycetes to change from a condition with apically flagellate pyriform zoospores to one of reniform laterally flagellate zoospores. This is brought about by the interpolation of an encysted condition. In the more advanced types the production of the first type of zoospore disappears completely, and the second stage takes place within the sporangium, or in a vesicle formed as an outgrowth from it, so that the only motile stage is the third. Finally we see that this motility is lost and the spores germinate by a germ tube within the sporangium.
In Saprolegnia, after the liberation of the zoospores from the zoosporangium, secondary sporangia may be developed. This may be effected, either by the development of a second sporangium from the base within the primary one, or by the production of a hypha from the same point, which grows up through the opening in the tip of the primary sporangium to form a secondary sporangium, sometimes some \Yay above the first. This is termed proliferation. Several successive generations of zoosporangia may be formed in this way (Fig. 203).
In addition to true asexual reproduction most species are capable of producing vegetative reproductive bodies termed gemmae which are developed from the tips of the hyphae. They are unicellular bodies, of varying shape, often globose or ovoid, but occasionally quite irregular. They are provided with dense protoplasm and stored food material, and are detached when mature. They germinate by the formation of a hypha which grows into a fresh mycelium. Little is known about the conditions which favour the production of gemmae, but they are partly hereditary, as it is not uncommon to find cultures which persist in forming these structures to the exclusion of any other type of reproductive body.
Sexual Reproduction
The sex organs consist of oogonia and antheridia. The oogonia are borne at the ends of long hyphae, or laterally on short branches, or are occasionally intercalary. They arise in the same way as the zoosporangia, by the contents of the hyphae migrating into the tip, after which it is cut off by a septum. The oogonia are generally spherical. The wall of the oogonium is usually smooth but provided with pits, though in some species it is covered with spines or papillae. The contents of the oogonium divide up into a varying number of parts. Rarely one oosphere is formed, but in most species from two to as many as a hundred oospheres may be produced within the oogonium. These oospheres are spherical and dark in colour, being richly supplied with oil, which serves as a good reserve. This oil is at first found as tiny droplets which coalesce to form a single drop which may either lie in the centre of the oosphere (centric), or may lie to one side of the oosphere (excentric). The oogonium is at first multinucleate and has dense cytoplasm. A central vacuole develops and the cytoplasm and nuclei are pushed out to the periphery, where the nuclei undergo a single mitosis and the majority of the daughter nuclei degenerate. Oospheres now form by the concentration of the cytoplasm around the remaining nuclei, each oosphere enclosing one nucleus.
The antheridia may arise either from the same branch as the oogonia, in which case they are said to be androgynous, or from entirely different hyphae, when they are termed diclinous. The antheridium is a narrow tube containing a rich supply of protoplasm, and is divided from its hypha by a septum; frequently it may be profusely branched.
It grows towards the oogonium, and finally becomes closely adpressed, laterally, against its wall. A very fine outgrowth of the antheridium, the fertilization tube, penetrates the wall of the oogonium, generally through a pit, and makes its way to an oosphere. Each oosphere is fertilized by a different antheridium or by a separate branch from the same one. The contents of the antheridium pass into the oosphere through this fertilization tube, and the male and female nuclei fuse. A thick wall is secreted around each oosphere, which thus becomes an oospore. The oospores are liberated by the breakdown of the oogonium. It seems probable in the case of oogonia with many oospheres that not all of them become fertilized.
Since it is very rare to find any oospheres lying among mature oospores it seems likely that those which fail to be fertilized develop parthenogenetically. In any case it is well known that in many species antheridia are either only rarely developed or not at all. In such instances, parthenogenetic development of the oospheres appears to be the rule. The oospore germinates by the formation of a germ tube which eventually produces fresh zoosporangia.
With one or two exceptions all the species of Saprolegnia are saprophytes on dead organic material, but Saprolegnia parasitica lives as a parasite on the gills of fish, causing the well-known Salmon Disease. This disease frequently attacks goldfish kept under unfavourable conditions in small tanks. It has been recently shown that excess of Calcium in the water favours the development of the fungus and makes the fish more liable to attack.
