Phyllum Thallophyta - The Algae - Phycomycetes - Peronosporales - Pythium debaryanum (The" Damping-off" Fungus)
Unlike Saprolegnia, Pythium debaryanum can live as a parasite within the tissues of seedlings, causing them to collapse and producing a diseasewhich is frequently spoken of as " damping-off". During the early stages in the growth of the Fungus no external signs of its presence are noticeable on the host, and it is only after it has gained a firm hold that the hypocotyl collapses, with the appearance of having been pinched off near the base. With the death of the host the Fungus continues to live saprophytically upon
the dead remains, and by means of external hyphae and zoospores attacks other seedlings near by. The disease spreads rapidly, and in a comparatively short time large numbers of seedling plants are killed. Pythium debaryanum also lives as a saprophyte in the soil, remaining there indefinitely before once again becoming parasitic on some higher plant. It should be noted that many other species of Pythium also cause disease among higher plants.
Pythium consists of long branched coenocytic hyphae, transverse septa being absent, except in association with the sex organs. The contents of the hyphae consist of a vacuolated mass of protoplasm in which nuclei are embedded. The infecting hypha may find its way into the host through a stoma, or may gain an entrance by dissolving the cell wall, probably by enzyme action. Once inside it grows rapidly, branching and forming an extensive mycelium. Although for the most part the hyphae remain in the intercellular spaces of the stem, they have the power of penetrating into the cells.
sually the mycelium is restricted, at any rate in the early stages of the disease, to the cortical region of the hypocotyl just above soil level ; the cells of the host die and collapse at this point and the seedling falls over. Later the hyphae make their way into the vascular bundles, where they absorb food substances which are being carried to different parts of the plant .. It is the appropriation of this food and the formation of waste products by the Fungus which lead to the death of the host and so cause the Fungus to become saprophytic. Long before the death of the seedling, however, reproductive organs are developed. Reproduction may be either by asexual or sexual methods.
ASEXUAL REPRODUCTION
The asexual reproductive organs are formed almost exclusively on aerial hyphae. Such hyphae are produced from a mycelium which grows out from inside the host, in the presence of a damp atmosphere, which favours growth
of the Fungus and also the production of zoospores. The tip of a hypha swells up and develops into a zoosporangium, which is then cut off by a septum. When the zoosporangium is mature a papilla is formed either at the apex or from the side of the sporangium. It is at first tubular, but later swells out at its distal end to form a vesicle. Into this the contents of the sporangium pass in a partly differentiated state, and in the papilla
zoospores are formed. They are reniform with two lateral flagella. The vesicle then breaks and the zoospores swim away in the water film. It sometimes happens that the sporangium functions as a conidiospore, and instead of forming zoospores grows out directly into a germ tube which produces a m:w hypha. In this way a new mycelium develops without the intervention of swimming spores. This provision enables reproduction to take place under relatively dry conditions where there is insufficient moisture for zoospores to swim in.
SEXUAL REPRODUCTION
The sex organs are developed within the tissues of the host, and consist of antheridia and oogonia. They appear at a late stage in the lifehistory of the Fungus, when the food material of the host is nearly exhausted. The oogonium is developed from the tip of a hypha into which some of the protoplasm, together with a number of nuclei, migrate and are cut off by a septum. The tip now becomes svvollen, and the protoplasm separates into a central granular portion, the oosphere, and a peripheral layer, the periplasm.
All except one of the included nuclei migrate to the periphery and abort, so that only one functional female nucleus remains in the centre.
Meanwhile the antheridium is developed as a club-shaped branch, either on the same or on an adjacent hypha, and is also cut off by a transverse wall. The antheridium now elongates, its tip comes into contact with the wall of the oogonium, which it dissolves at this point, and a fertilization tube is formed which penetrates the oogonium and passes into the oosphere. It is probable that only a single male nucleus enters the oogonium, and this immediately fuses with the female nucleus. The fertilized oogonium secretes a thick wall round itself, and thus becomes an oospore. In the oospore are a large number of oil globules \yhich constitute the food resen'e of the zygote. The oospore is a drought-resisting structure, associated with the dessication which the Fungus may experience. The germination of the oospore only takes place in the presence of moisture and may be delayed
almost indefinitely. The thick wall of the oospore bursts, and the contents, surrounded by a thin membrane, may either grow out directly into a hypha, \yhich immediately forms a fresh mycelium, or, under other conditions, this hypha may bear an asexual sporangium. Occasionally, however, the development of the hypha is suppressed, and zoospores similar to those which would have been borne in an asexual sporangium are developed directly within the oospore. These swim about in the water surrounding the host, until they find fresh tissues to penetrate, when they form a germ tube which develops into a hypha.
The life-cycle of Pythium is graphically represented thus ;-
It is important to realize that a large number of species of Pythium occur in this country, many of them in the soil, and that 'vvhile some are destructive parasites of higher plants others may play an equally important part in the decomposition of plant remains in the soil. These species show a considerable nriation from the type described above. For example, in some the oogonium may be beset with spines, while in others the chief differences are in the formation of the zoosporangia and the germination of the zoospores. In fact it is possible to trace certain evolutionary sequences indicating a possible gradual evolution from a typically aquatic mode of life to one adapted to terrestrial conditions among the various species of the genus. A similar sequence is found in the genus Peronospora, and we will postpone our account of these matters till after we have described the life-history of this organism.
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