pycnidium), which are flask-shaped, and acervuli (singular: acervulus), which have a simpler cushion-like form. Pycnidial conidiomata or pycnidia form in the fungal tissue itself, and are shaped like a bulging vase. The conidia are released through a small opening at the apex, the ostiole. Acervular conidiomata, or acervuli, are cushion-like structures that form within the tissues of a host organism: subcuticular, lying under the outer layer of the plant (the cuticle), intraepidermal, inside the outer cell layer (the epidermis), subepidermal, under the epidermis, or deeper inside the host. Mostly they develop a flat layer of relatively short conidiophores which then produce masses of spores. The increasing pressure leads to the splitting of the epidermis and cuticle and allows release of the conidia from the tissue. Health issues Conidia are always present in the air, but levels fluctuate from day to day and with the seasons. An average person inhales at least 40 conidia per hour. Exposure to conidia from certain species, such as those of Cryptostroma corticale, is known to cause hypersensitivity pneumonitis, an occupational hazard for forest workers and paper mill employees. Conidia are often the method by which some normally harmless but heat-tolerating (thermotolerant), common fungi establish infection in certain types of severely immunocompromised patients (usually acute leukemia patients on induction chemotherapy, AIDS patients with superimposed B-cell lymphoma, bone marrow transplantation patients (taking immunosuppressants), or major organ transplant patients with graft versus host disease). Their immune system is not strong enough to fight off the fungus, and it may, for example, colonise the lung, resulting in a pulmonary infection. See also Arthroconidium Ascocarp Basidiocarp Budding Gemma Phialide References External links Fungal morphology and anatomy