Ascomycotina is the largest division of fungi. If lichens are included, there are over 42 000 species. In spite of a considerable range of form, the division appears to be a natural one. Its members are characterized by having asci. The ascus is a special type of sporangium which usually contains eight ascospores. Development is highly distinctive. The young ascus has two haploid nuclei. These fuse to form a diploid one which immediately undergoes meiosis so that four haploid nuclei are formed. Usually each of these then divides mitotically. Around the individual nuclei ascospores are organized, but some protoplasm (epiplasm) is left outside the eight ascospores (Fig. 4.1). In most Ascomycotina the ascus bursts at maturity, squirting its contained spores into the air to a distance varying with the species concerned from 2–400 mm (Fig. 4.2 and Plate 1d).
KeywordsPowdery Mildew Black Crust Spore Discharge Female Organ Haploid Nucleus
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