Phylum Thallophyta - The Algae - Ascomycetes - Plectascales
The Plectascales are Ascomycetes in which the product of the fertilization of the ascogonium is frequently only a single ascus. In some of the higher members, where more than a single ascus is produced, the asci are not arranged in any system, but are scattered irregularly in a special closed body termed the perithecium.
Consider one example of this order, Aspergillus.
Aspergillus (Eurotium) herbariorum
This Fungus is a saprophyte on fatty substances and forms one of the most frequent contaminants of food. Some species are parasitic on man and animals, but the majority live as saprophytes. The mycelium consists of filaments of multinucleate cells, and ramifies over the surface of the substratum on which it lives, sending up vertical hyphae ,yhich bear the asexual reproductive organs. The name Eurotium was originally applied to the sexual stage, while the name Aspergillus was used for the asexual. This was due to the fact that until comparatively recently the two stages were regarded as those of distinct Fungi, owing to the fact that either stage could apparently continue indefinitely without the appearance of the alternative stage. This tendency among Fungi for the asexual stage to continue generation after generation without the intervention of a sexual stage we have already met with in Mucor. It is equally true of many of the Ascomycetes, in fact there exists a large group of Fungi, termed the Fungi Imperfecti, which includes many important plant pathogens, and comprises Fungi in which only the asexual stage is known.
ASEXUAL REPRODUCTION
The asexual reproductive bodies are borne on conidiophores These arise as vertical hyphae which grow out from the mycelium. They are thick and are not divided by septa. The tips of these conidiophores
become swollen and bud out numerous small processes, the sterigmata, from each of which conidiospores are produced in chains in acropetal succession. The spores are distributed by air currents, and on coming to rest on a suitable substratum each germinates to produce a hypha from which a fresh mycelium develops.
SEXUAL REPRODUCTION
The archicarp or fruit primordium consists of three parts: a multicellular stalk, an elongated unicellular ascogonium, and a unicellular trichogyne. The antheridium arises from the same initial hypha as the 42.-Aspergillus herbariorum. A to D, Stages in the development of the perithecium around the sex organs. E, Section through perithecium showing ascogenous hyphae. F, A single ascus containing ascospores. (After de Bary.) ascogonium, or from an adjacent one. It is unicellular and multinucleate. Fusion takes place between the antheridium and the trichogyne, which become twisted around one another during the process. It has not been proved, however, that the antheridium is ever functional in fertilization. In some species the antheridium may be absent, but this does not prevent the further development of the ascogonium. In either case the ascogonium becomes divided into a number of binucleate cells, from each of which a septate ascogenous hypha is given off. From the terminal cell of each hypha or from the cell below, termed the penultimate cell, an ascus is formed by elongation. In this cell the two nuclei fuse together.
Shortly after the development of the sex organs, hyphae begin to grow up around them and the whole structure becomes enclosed in a t\yo-layered envelope, the perithecium, the inner layer of which forms a nutritive tissue, while the outer serves for protection. This latter tissue secretes a bright yellow substance ·which makes the sexual reproductive bodies easily recognizable.
After the fusion of nuclei \yithin the ascus the zygote nucleus so formed divides meiotically into four, and this is followed by a further mitotic division. Around each of these eight nuclei a wall is laid down forming an ascospore. These spores have a sculptured epispore wall and are rounded in shape. They are liberated upon the decay of the perithecium surrounding the reproductiw organs. They germinate to produce fresh mycelium.
Closely allied to Aspergillus is the genus Penicillium, which is used, associated with other Fungi, in the "ripening" of cheese. It differs chiefly from Aspergillus in the form of the conidiophore. The conidiophores are narro,,· and branched and not swollen at their tips, while the conidiospores are abstricted in more or less parallel chains. The sexual reproduction is similar to that of Aspergillus. The wellknown antibiotic substance Penicillin is a product of Penicillium notatum.
