140 PLANT MORPHOLOGY 



the perithecia, lacking independent walls, are developed as stromatal 

 cavities. Plowrightia belongs here. 



3. Sphacriales. The Sphaeriales have dark-colored perithecia with a 

 distinct wall. The perithecia may be free or embedded in the substratum 

 or in stromata that are firm, leathery or brittle, and dark-colored. Here 

 belong Venturia, Xylaria, and Neurospora. 



9. Laboulbeniales 



The Laboulbeniales comprise an order of about 50 genera and 1,200 

 species. They are parasitic on insects, especially aquatic ones. As a rule 

 the mycelium grows on the surface of the host and is very small, usually 

 less than 1 mm. in length. The Laboulbeniales are of particular interest 

 because their sex organs are remarkably like those of the red algae. The 

 antheridium is unicellular and produces a nonmotile male cell, the 

 spermatium. The ascogonium has a trichogyne and auxihary cells. 

 Ascogynous hyphae arise from the fertilized ascogonium, small perithecia 

 are formed, and the asci bud out from the auxiliary cells. The whole 

 process resembles cystocarp formation in the red algae. 



5. BASIDIOMYCETES 



The Basidiomycetes, or club fungi, comprise the highest group of fungi. 

 They resemble the Ascomycetes in having a mycelium with cross walls. 

 They are characterized by the production of spores externally on a club- 

 like structure known as a hasidiiOm. This arises from the swollen end of 

 a hypha and may consist of either four cells or one. Four slender branches 

 (sterigmata) arise from the basidium, each forming a hasidiospore at its 

 tip. The young basidium contains a nucleus derived from the fusion of 

 two nuclei. Two successive divisions, which are reductional, result in the 

 formation of fourhaploid nuclei, each passing into one of the basidiospores. 

 In the higher members the basidia are borne on a distinct fruiting body, 

 the hasidiocarp, composed of interwoven hyphae. The Basidiomycetes 

 are related to the Ascomycetes and are generally regarded as having been 

 derived from them. Some are of great economic importance, particularly 

 the smuts, rusts, and mushrooms. The Basidiomycetes number about 

 20,000 species. They embrace seven principal orders: Ustilaginales, 

 Uredinales, Auriculariales, Tremellales, Exobasidiales, Hymenomycetales, 

 and Gasteromycetales. '^ 



1. Ustilaginales 



The Ustilaginales, or smuts, are parasites that live on various her- 

 baceous seed plants. They attack chiefly floral organs, particularly those 

 of grasses. They are most destructive to oats, less so to wheat and corn. 

 The smuts number about 500 species. The principal genera are Ustilago 

 and Tilletia. 



