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PLANT MORPHOLOGY 



known as peach leaf curl, while Taphrina pruni produces a disease of the 

 domestic plum called plum pockets, in which the fruit becomes shriveled. 

 Taphrina cerasi attacks branches of the cherry, causing brush-like deform- 

 ities known as witches'-brooms. 



The mycelium of Taphrina grows in the intercellular spaces of the 

 host and sends to the surface groups of asci that arise just beneath the 

 cuticle (Fig. 97). Each ascus contains eight ascospores. The asci are 

 crowded to form a layer, called the hymenium, but are Avithout accom- 

 panying sterile hyphae. Moreover, an ascocarp is not developed and 



Fig. 97. Taphrina deformans. Cross section of portion of peach leaf, showing layer of asci 

 and ascogenous cells on the surface, X500. 



there is no formation of sex organs. The cells of the mycelium are 

 binucleate. The two nuclei in the young ascus fuse, three successive 

 divisions result in the formation of eight free nuclei, and from these the 

 eight ascospores are organized. Upon germination, the ascospores, which 

 are haploid, may give rise to one or more uninucleate cells by a process 

 that resembles budding in yeasts. In some species these cells, which 

 are called conidia, are formed while the ascospores are still within the 

 asci. The ascospores, or the conidia produced by them, infect new host 

 plants, a hypha penetrating the cuticle and pushing its way between the 

 epidermal cells. The germinating spore may become binucleate by divi- 

 sion of its nucleus, or a pair of ascospores or conidia may conjugate, a 

 nucleus passing from one to the other. The binucleate condition is then 

 transmitted to the cells of the vegetative mycelium. 



3. Plectascales 



The Plectascales include the blue and green molds, saprophytes that 

 are abundant everywhere, occurring on bread, cheese, jelly, fruits, vege- 

 tables, meat, leather, etc. The order includes over 30 genera and 800 

 species. The two commonest genera are Aspergillus and Penicillium, 

 the latter numbering over 500 species. One species, Penicillium notatum, 



