HEMIBASIDIOMYCETES 373 



per cent. This estimate would mean a loss of about twenty million 

 dollars, based upon the statistics of oats produced during 1906. 



The mycelium of the oat smut is present throughout the tissues 

 of the affected plants. Infection takes place by means of the germi- 

 nating conidia at the time of germination of the seed. The mycelium 

 branches abundantly in practically all tissues of the developing 

 flowers, completely infesting the young ovule and the inclosing 

 floral structures. The mycelium, at the time that spore formation 

 becomes evident, shows a nodulate appearance, and the branches 

 are closely fascicled, like clusters of grapes. Within each swollen 

 area of the mycelium a chlamydospore is found. As the chlamydo- 

 spores mature, the inclosing walls of the parent hyphae and much 

 of the general mycelium which is not differentiated into spores 

 gelatinizes or otherwise breaks away, and the spores are set free 

 in large masses. With the increased growth of the mycelium and 

 the formation of spores, the softer cells of the host plant are rapidly 

 absorbed, so that at maturity only the more resistant tissues of the 

 florets may remain, the whole ovule with its inclosing glumes being 

 largely converted into the dusty mass of sooty spores. In a closely 

 related species of oat smut (Ustilago levis), long regarded merely 

 as a race or variety of Ustilago Avchcb, the mycelium destroys only 

 the kernels and does not attack the glumes. The smut therefore 

 remains inclosed or hidden. 



The spores are almost spherical or slightly ovoidal, and echinu- 

 late, varying in length from 5 to 9^. They are also olivaceous 

 in color, with a lighter area at one side. Germination of the fresh 

 or of preserved spores may be readily secured. In fact, in herbarium 

 material spores may preserve their vitality for several years. Ger- 

 mination may proceed in pure water or in nutrient solution. 



The promycelium is frequently four-celled, though somewhat 

 variable in this regard, and it often assumes abnormal forms, as 

 shown in Fig. 190. The conidia are produced laterally and termi- 

 nally. They are elliptical or subelliptical in form and measure 

 4.5-8 x 4.5-6 /a. In nutrient solutions the well-known budding 

 of the conidia may continue almost indefinitely, and under certain 

 conditions, or after extensive cultivation, mycelium-like cells may be 

 produced. Upon the living host, however, the conidia germinate 

 by the production of an infection hypha. 



