126] The Classification of Lower Organisms 



The hyphae of Ascomycetes are septate and the cells most often uninucleate. 



Most Ascomycetes produce, beside the ascospores, conidia of one type or another. 

 A mycelium may produce a mass of densely woven hyphae with conidia on the sur- 

 face; such a mass is called an acervulus or sporodochium. Either a mycelium or an 

 acervulus or sporodochium may send up spore-bearing columns called coremia. 



Many Ascomycetes produce, either directly from the mycelium or from special 

 structures consisting of interwoven hyphae, globular or flask-shaped structures which 

 produce conidia internally and release them through a pore. These structures are 

 called pycnidia, and the spores pycniospores. In many examples, the pycniospores 

 are capable of functioning as sperms; so far as this is true, the pycnidia may alter- 

 natively be called spermagonia, and the pycniospores spermatia. 



Hyphae woven into a mass may go into a resting condition, becoming thick-walled, 

 hard, and usually dark in color. The resulting structure is a sclerotium. If a structure 

 of the general nature of an acervulus, sporodochium, or sclerotium gives rise either 

 to pycnidia or to fruits bearing asci, it is called a stroma. 



As to asci and ascospores, Dangeard (1893, 1894, 1907) reached definitely the 

 conclusion that they are essentially sexual products. There had been earlier observa- 

 tions, beginning with de Bary, 1863, that there are meetings, coilings together, and 

 fusions of hyphae as a preliminary to the production of asci. Many ascomycetes are 

 of two mating types; this was first discovered of Glomerella, by Edgerton (1914). As 

 Dodge (1939) remarks, the mating types are not sexes; in forms producing recogniz- 

 able male and female reproductive structures, each mating type may produce both. 



In Ascomycetes which may be regarded as primitive, difTerentiated male and fe- 

 male cells are produced. The male cell or antheridium is ordinarily terminal on a 

 hypha. The female cell (constituting, together with other differentiated cells of the 

 same hypha, if any are present, the ascogonium) may be terminal; more often it bears 

 an elongate cell, or a chain of cells, called the trichogyne, and having the function 

 of reaching the antheridium. In some Ascomycetes, antheridia are produced, but syn- 

 gamy does not take place; the egg is binucleate or multinucleate, and the nuclei 

 within it take the part of gamete nuclei in further development. There are others in 

 which no antheridia are produced. Hansen and Snyder (1943) found, in Hypornyces 

 Solani var. Cucurbitae, that "any part of the living thallus, ascospores, conidia or 

 bits of the mycelium could act as the male fertilizing agent." There are forms in 

 which fusions take place between undifferentiated hyphal cells; and yet others in 

 which it appears that the paired nuclei involved in sexual processes arise by divisions 

 of a single nucleus originally present in a spore. 



In some Ascomycetes, syngamy is followed immediately by karyogamy, and the 

 zygote develops directly into a single ascus. In the overwhelming majority of the 

 group, asci are produced indirectly, and there is no fusion of nuclei until this takes 

 place. The zygote sends out hyphae called ascogenous hyphae, recognizably different 

 from the vegetative ones. The cells of the ascogenous hyphae arc binucleate; or, 

 arising from a multinucleate zygote, become binucleate by the establishment of 

 crosswalls. The two nuclei of each cell divide concurrently and the cell walls are so 

 placed that each cell receives nuclei of different origin. This effect is achieved in the 

 final cell division before ascus formation by a peculiar process called crozier forma- 

 tion. The terminal cell of the ascogenous hypha becomes bent to the form of a hook; 

 the nuclei divide concurrently, and cell walls appear between the daughter nuclei of 

 each pair; the middle cell of the row of three thus produced remains binucleate and 

 becomes an ascus. The uninucleate terminal and basal cells lie side by side, and may 



