HYPOCREALES 



229 



In contrast to all these genera, Poly stigma is parasitic on angiosperms. 

 Its best-known representative, P. rubrum, causes a red spot disease of 

 Prunus domestica, P. insititia and P. spinosa (Blackman and Welsford, 

 1912; Nienburg, 1914). The ascospores germinating in the spring infect 

 the young leaves bursting from the buds and grow to a large intercellular 

 mycelia. The hyphal cells are uni- to trinucleate and contain an 

 orange-yellow, alcohol-soluble pigment; their walls are at first thin, later 

 becoming thick and gelatinous. In 5 or 6 weeks they fill the whole space 

 between the epidermal layers and during the summer form shining, red- 

 dish yellow or scarlet sclerotic stromata. 



Before this development has proceeded far, flask-shaped pycnidia 

 are formed from knots of unthickened hyphae under the stromata. 



Fig. 147. — Polystigma rubrum. 1. Section through portion of leaf with young pyc- 

 nium. 2. Section through young perithecial fundament showing helical ascogonium. 

 3. Pycnospores. (1 X 200; 2 X 600; 3 X 1,200; after Blackman and Welsford, 1912.) 



The pyenidial wall consists of a plectenchyma from which the sporiferous 

 hyphal branches radiate (Fig. 147, 1). The pycnospores are terminal, 

 uninucleate, tapering upward and falcate at their tips (Fig. 147, 3). 

 They are embedded in a slimy substance and in damp weather are forced 

 out of the mouth of the pyenia upon the leaf. 



Later, in July or August, the perithecium is formed, as a small hyphal 

 tangle in which is embedded a helical ascogonium (Fig. 147, 2). Its 

 development is incompletely known; a half schematic cross section of an 

 older stage is shown in Fig. 148, 1. The basal cell of the ascogonium 

 bordering on the ascogonial hypha is short and has few nuclei. Adjacent 

 to it is an elongated cell with many small nuclei. The next cell has only 

 a single, rather large nucleus but is much shorter than both the previous 

 ones. There follow two cells with two small nuclei each; then the helix 

 continues with cells with an irregular number of nuclei and finally is lost 



