172 



BOTANY 



cells of the perithecium there are developed curious appendages, 

 upon the form of which the genera are largely based (Fig. 137, A). 



Of the higher Pyrenomycetes, some are parasites, others sapro- 

 phytes, usually growing on dead wood, leaves, etc. Xylaria, Cordy- 

 ceps, Pleospora, Sordaria, are among the common genera. The large 

 black masses of Xylaria are sometimes very conspicuous upon dead 

 wood. Cordyceps is a parasitic genus, some of whose species, e.g. 

 C. militaris, attack insects, especially caterpillars, which are killed 

 by them. Claviceps purpurea causes the disease known as " Ergot " 

 upon Rye. In many of these the ascospores are multicellular. 



F 



FIG. 139. Black-knot (Plowrightia morbosa). A, Plum twig attacked by Black- 

 knot. (Natural size.) B, conidia. C, stylospores. D, section of perithecium, 

 showing asci and paraphyses. E, single ascus, with two paraphyses, more highly 

 magnified. F, germinating ascospore. (B-F, after FARLOW.) 



As a type of the higher Pyrenomycetes, we may select a very striking form of 

 the eastern United States, Plowrightia morbosa, which causes the destructive 

 disease of plums and cherries, known as "Black-knot." The mycelium grows 

 within the tissues of the younger twigs, where it produces unsightly rough swell- 

 ings which give the disease its common name. 



In the spring the mycelium shows active growth, and breaks through the outer 

 layers of the bark, upon which it produces dense masses of conidia, borne upon 

 rather thick-jointed conidiophores (Fig. 139, B). Thisconidial form was at first 

 described as a distinct genus under the name of Cladosporium. 



As the summer advances the knot grows larger, and in its outer part may be 

 detected the young perithecia, which were present, however, earlier in the sea- 

 son. These form little papillae with a pore at the apex opening into the cavity 

 within. Late in the autumn the young asci, intermixed with paraphyses, may 

 be seen lining the perithecia, but the asci (E) are not ripe until midwinter. 



