128] The Classification of Lower Organisms 



hypotheses, long accepted as possible, were disproved by genetic studies by Betts and 

 Meyer (1939) and Keitt and Langford (1941). In the asci of many species, the 

 spores lie in a single series in which their order is determined by the divisions which 

 produce their nuclei. By refined technique, the spores from a single ascus may be 

 identified, separated, and cultivated. It is then observed that the mycelia grown from 

 the first four spores may differ in some particular character from those grown from 

 the second four spores; those from the first pair of spores may differ from those from 

 the second; but those from two members of any of the pairs, first, second, third or 

 fourth, are always alike. These observations mean that the first two divisions in the 

 ascus constitute the meiotic process, the third being mitotic. Lucas (1946) obtained 

 cytological evidence refined enough to confirm this conclusion. 



Asci are almost always produced in fruits, which may be called ascocarps. The 

 ascocarp aside from the asci arises usually from vegetative hyphae; in the Ascomy- 

 cetes regarded as primitive, it does not begin to develop until after fertilization, but 

 in the higher ones it may develop in advance of fertilization and become the seat 

 of this process. 



There are several types of ascocarps, among which three are most familiar. A 

 small ascocarp completely enclosing the asci is a cleistothecium. Cleistothecia were 

 formerly included under the term perithecium; that term will better be limited 

 to small fruits which are globular or vase-like, opening through a single pore, the 

 ostiole, and differing from the pycnidia already described in producing ascospores 

 instead of conidia. A fruit in which the asci form a broad layer which is typically 

 fully exposed at maturity, the whole being ordinarily of the form of a disk or cup, 

 larger than a cleistothecium or perithecium, is an apothecium. 



Asci produced in perithecia or apothecia usually discharge the ascospores vio- 

 lently. The mechanism of discharge is apparently simply turgidity. Some asci show 

 no visible adaptations for the discharge of spores; others have lids (opercula) 

 whose position determines the direction of discharge. Certain large apothecia can 

 throw the spores to a distance of 10-20 cm.; the discharge is so governed by tempera- 

 ture and humidity as to occur in gently moving rather than in still air. By blowing 

 across these apothecia one can make them throw out a visible cloud of spores. 

 Heald and Walton (1914) reviewed many older observations of violent discharge by 

 perithecia, the oldest by Pringsheim on Sphaeria Scirpi, 1858. Rankin, 1913, found 

 that each ascus in turn breaks loose, comes up to the ostiole, projects through it, 

 throws out its spores, and collapses to make room for another. Weimer (1920) found 

 that the perithecia of Pleurage curvicolla bend toward the light and throw the spores 

 to a maximum distance of 45 cm., which is apparently the record. 



There is a widely entertained hypothesis that the Ascomycetes evolved from the 

 red algae. It appears to have developed from a piece of classification by Sachs 

 (1874), who proposed a class Carposporeen, to consist of the red algae, certain higher 

 green algae, and the Ascomycetes and Basidiomycetes. A number of resemblances 

 support it. Both red algae and Ascomycetes include many parasites; both lack 

 flagellate cells; both have differentiated gametes, the egg bearing a trichogyne; in 

 both, fertilization leads to further development before spores are produced. In 

 addition to these genuine resemblances, an imaginary one was influential, namely 

 the double fertilization ascribed to the red algae by Schmitz and to the Ascomycetes 

 by Harper. Numerous as these resemblances are, they are not now believed to indicate 

 relationship. Atkinson (1915) formulated the counter-argument. The Ascomycetes 

 resemble the Mucorina in nutrition, in producing no flagellate cells, and in multi- 



