Phylum Thallophyta - The Algae - Ascomycetes - Phacidiales
The Phacidiales are Ascomycetes in which the apothecia are immersed in the fungal tissue which is partly buried in the host tissue. They are generally small in size and of a leathery or corky consistency. The group is a small one but includes several genera which produce spots on the leaves of higher plants. It should be noted, however, that many of the common leaf-spot diseases are not caused by members of this order. Some of these diseases are caused by members of the Sphaeriales in which the ascocarp is flask-shaped, whereas in the Phacidiales it is more or less flat and widely open. Others, and indeed the majority of leaf-spot Fungi, are placed among the Fungi Imperfecti because they reproduce only asexually, by conidiospores.
Consider one example of this order, Rhytisma acerinum.
Rhytisma acerinum (The Black Spot Disease of Maples)
This Fungus is extremely common throughout the country, attacking the leaves of the common Field Maple and also the Sycamore; in fact, few trees and indeed few leaves escape. Superficially these leaves appear to have been spotted with large drops of tar, and it seems that the lower leaves are usually more frequently attacked and have larger spots than those at the top of the trees. These black spots occur on the upper surface and average about 15 mm. in diameter. They consist of a wrinkled stroma, roughly circular in shape, which is formed mainly in the epidermal cells of the leaf, though the hyphae spread down into the cells of the palisade layer as well.
The mycelium arises from a germinated ascospore which forms a germ tube and enters the leaf through the epidermis. It traverses the intercellular spaces of the palisade cells and also the spongy mesophyll.
In the epidermal cells the mycelium penetrates the cross walls repeatedly, so that the outer surface of the epidermal cells gradually becomes split away from the rest of the leaf. Meanwhile black material becomes deposited in between the epidermal cells, and the infected area changes colour from yellow to black.
ASEXUAL REPRODUCTION
At a fairly early stage in development it is possible to see the elaboration of the future asexual reproductive area by the modification of the part between the upper and lower walls of the epidermal cells (Fig. 263). Not only does the deposit of black material become denser along the lower surface but the mycelium gradually forms a continuous tissue in the bases of the cells and produces vertically growing hyphae, which extend upwards towards the upper walls of the epidermal cells. These elongating hyphae are really conidiophores and later abstrict conidiospores.
Meanwhile the mycelium from which the conidiophores arise becomes more dense and forms a closely interwoven tissue of small uninucleate cells termed a plectenchyma. Kot only does this form the entire floor of the cavity, but circular ramparts grow up, dividing the area into a number of cavities which are for a time covered by the upper walls of the original epidermal cells, now completely disorganized. As these cavities enlarge vertically the conidiophores commence to cut off conidiospores in chains and fill the cavities, which eventually burst open by the breakdown of the upper epidermal walls. Such cavities, in which asexual conidia are produced, are termed acervuli. Despite the fact that these conidiospores are produced in vast numbers they have not been germinated under experimental conditions, and there is no evidence that they do actually spread the Fungus during the summer months when they are mainly produced.
SEXUAL REPRODUCTION
The apothecia may be formed either in old acervuli or in newly formed cavities in the epidermis. During the autumn the plectenchyma becomes more sclerotic and blackens considerably. In the interior of the plectenchyma numerous apothecial cavities become differentiated. These become filled with loose hyphal tissue and are covered not only by the outer epidermal walls but also by a thick-\\'alled black tissue called the epithecium. Below, the cavities rest on a somewhat lighter, thick-walled tissue, the hypothecium, which in turn covers the lower epidermal walls.
In each of these apothecial cavities the female sex organs are formed.
Each consists of a uninucleate stalk cell, two or three multinucleate ascogonial cells and a uninucleate trichogyne. Apparently no male sex organ is formed and no true fertilization takes place. Instead, the septa between the ascogonial cells break down and their nuclei pair together.
Ascogenous hyphae now grow out from the ascogonium and paired nuclei migrate into them. The ascogenous hyphae branch repeatedly, and many asci are produced from a single ascogenous hypha. Each young ascus possesses a pair of nuclei derived by conjugate division from the original pair in the ascogenous hypha. These two nuclei then fuse.
The single fusion nucleus in the ascus now undergoes three divisions, resulting in the formation of eight nuclei, around which eight ascospores are formed. These ascospores are long and slender and lie side by side in a packet along the length of the ascus.
Meanwhile, from the loose hyphae in the apothecial cavity paraphyses are differentiated which grow up between the asci, so that at maturity the asci are more or less enveloped in a mass of very slender, elongated filaments.
The mechanism for bursting the apothecium is elaborate. Changes, mainly of the roof of the apothecial cavity, are initiated about the time that the asci begin to form. A layer of cells about three-quarters of the depth from the top break down, thus forming a small rift which gradually expands downwards, while at the same time the upper part of the roof rises and finally ruptures at the top. At this stage the apothecium is closed only by the tissue below the rift. When the spores are ripe this too ruptures and the apothecial cavity is exposed. Some apothecia never open widely and the opening remains as a narrow fissure, in others the roof bends back so that the whole cavity is exposed.
Since the development of the asci takes place during the winter it must obviously occur when the leaves have been shed, hence the discharge of the ascospores will take place in the early spring at a time ,yhen the leaf remains have largeiy rotted and the bits are blown about. In this way additional distribution of the Fungus is effected quite apart from the fact that the ascospores are forcibly ejected from the ascus. Moreover, as a result of contraction of the epithecium the ascospores may be shot up as much as I mm. above the surface of the substratum. They ultimately reach the young leaves as they unfold i~ the spring and there germinate to start the life-cycle over again.
