Nov. 15, 1920 Crownwart of Alfalfa Caused by Urophlyctis alfalfae 311 



from the chromatin. Plate 49, D, shows a condition that frequently 

 appears in spores that probably have been poorly nourished. The degen- 

 eration of the central nuclei leads to the origin of a large vacuole that 

 ultimately develops into a cavity near the periphery of which a variable 

 number of red-staining granules are always to be found. 



Maturation involves, too, a conspicuous transformation and thicken- 

 ing of the wall of the resting spore. Even while growth is still pro- 

 ceeding, the spore wall becomes increasingly thick; and during the later 

 stages of enlargement, although still capable of further distension, in all 

 probability it no longer permits of an easy passage of food materials. 

 After final size is attained, thickening proceeds rapidly. The mature 

 spore wall is a structure about 1.5 n in thickness, of a yellow, vitreous 

 appearance, inelastic and brittle; when the wall is fractured by pressure 

 applied in manipulation, fragments may break out like pieces of shell 

 from a nut, often leaving the contents quite intact. 



When the spore has attained maturity, the haustorial processes dis- 

 appear, whether by retraction, degeneration, abscission, or accidental 

 fracture could not be definitely determined. However this may be, 

 a circle of pits or scars, corresponding in number and position to the 

 haustoria (PI. 48, F, G), is always left, because the thickening of the 

 spore wall never involves the places of attachment of the haustoria. 

 In examinations of herbarium material, in which turbinate cells and 

 hyphae are only too frequently quite unrecognizable, these pits serve as 

 a morphological feature of no mean taxonomic value. 



GENERAL, TAXONOMIC CONSIDERATIONS 



The taxonomic relations of the plants included under the genera 

 Urophlyctis, Physoderma, and Cladochytrium remain in need of study. 

 Schroeter (20) saw in the association of the "Oosporangium " of U. pulposa 

 with the "leere Blase" a sexual apparatus consisting of two conjugating 

 "Fruchtkorper," one of which has yielded its contents to the other. 

 On the basis of this interpretation he erected the genus Urophlyctis, 

 including it with Diplophysa and Polyphagus in the Oochytriaceae, 

 which family he distinguished from all the other families in the Chytri- 

 dineae not excluding the Cladochtriaceae, under which were brought 

 Physoderma and Cladochytrium by the presence of sexuality in the 

 origin of the resting spores. Fischer (9), on the other hand, denied the 

 existence of sexuality in Schroeter's genus and placed it with Physoderma 

 as a subgenus under Cladochytrium. Schroeter's views received support 

 from Magnus, who described a number of forms — U. kriegeriana (18), 

 U. leproidea (18), U. rilbsaameni (19), and U. alfalfae (20) — as con- 

 generic with U. pulposa and exhibiting the same type of oogamy. The 

 later investigations on U. leproidea by Vuillemin (55) , on U. rubsaameni 

 by Bally (<?), and on U. hemisphaerica by Maire and Tison (21) have 



