66 PHTCOMYCETEAE 



bursts off the upper side of the thick wall with a circular split, forming a 

 very large lid. On the thin inner wall an inoperculate exit papilla is 

 formed, setting free the posteriorly uniflagellate zoospores. The suggestion 

 may be made that possibly the externally formed zoospores are capable 

 of functioning as gametes, the external zoosporangia arising from zoospore 

 infection, the internal infection by zygotes. Physoderma zeae-maydis Shaw 

 is sometimes destructive to corn (maize, Zea mays L.) in the southern 

 parts of the United States and in Asia. Usually only the internal rhizo- 

 mycelium and the "Sammelzellen" and resting sporangia are observed 

 (Tisdale 1919) but Sparrow (1934, 1947) has shown that it is possible to 

 obtain the production of the external slipper-shaped sporangia by placing 

 pieces of young maize leaves in a hanging-drop culture with zoospores 

 from the resting sporangia. As in P. maculare new zoosporangia are 

 formed within the emptied older ones. These planospores are markedly 

 smaller than those arising from the resting sporangia and Sparrow sug- 

 gests that they may possibly be gametes. (Fig. 19.) 



Jones and Drechsler (1920) and also Bartlett (1926) have shown that 

 Urophlyctis has much the same life history as Physoderma but causes 

 extensive gall production by the host, this being practically the only 

 distinction between the two genera. The external zoosporangia have been 

 reported in several species of Urophlyctis. Within the infected epidermal 

 cell there develops a " Sammelzelle " or "turbinate cell," at first uni- 

 nucleate but soon multinucleate. At its distal end is formed a terminal 

 tuft of haustoria. At several places on the cell, buds are formed into each 

 of which a nucleus passes, following which the bud grows out as a some- 

 what enlarged end of a very slender non-nucleate filament. This enlarge- 

 ment in turn becomes a turbinate cell and may give rise to other similar 

 cells, usually three to five cells from each. In the center of the distal tuft 

 of haustoria there soon buds out a thin-walled cell which grows rapidly 

 and becomes much larger than the cell from which it originates. This 

 becomes thick-walled and bears a crown of haustoria. During its growth 

 the cytoplasm and nuclei from the turbinate cell pass into it. This is not 

 an act of fertilization, as was believed by earlier mycologists. The con- 

 necting rhizomycelium soon disappears and finally only the resting 

 sporangia are to be found in the gall tissue. After some time these spo- 

 rangia are capable of germination. Scott (1920) has studied this process 

 in U. alfalfae Magn. It produces one to fifteen or more zoosporangia 

 varying in diameter from 10 to 40 microns, which push out through 

 irregular fissures in the brown wall. Zoospores escape through short exit 

 papillae, there being several such papillae on the larger sporangia. The 

 zoospores are 4 to 8 microns long with a posterior flagellum 30 to 50 

 microns in length. Scott observed no conjugation of these zoospores. On 

 the other hand, 0. T. Wilson (1920) reports that the zoospores are 



