ALBUGINALES 101 



examination of sporangia from 42 cruciferous genera led them to 

 divide A. Candida into two groups, viacrospora and viicrospora. 

 A monograph by Wilson (1907, 1908, 1908a) deals with the 

 classification of species known to occur in North America. 



Albugo is characterized by the possession of sporangiophores 

 arranged in a palisade-like layer, thus forming pustules beneath 

 the epidermis of the host. The sporangiophores are broadly 

 clavate, and the sporangia are abstricted in basipetal succession 

 and adhere in chains. The sporangia have the appearance of be- 

 ing separated by intercalary cells, but instead the outer sporangial 

 wall partly gelatinizes at the point of contact of adjacent 

 sporangia, forming disks ^^•hich, when gelatinization is complete, 

 permit the sporangia to fall apart. Pressure exerted on the super- 

 imposed epidermis by the accumulation of sporangia finally 

 causes the leaf tissues to rupture and permits the powder\' mass 

 of sporangia to be disseminated by the wind. 



Germination of the sporangium results in the formation of 12 

 to 20 zoospores. The zoospores are reniform and bifiagellate. 

 After a brief period of swarming, they encyst, and further de- 

 velopment consists in the formation of a germ tube which is 

 capable of penetrating a suitable host by way of the stoma. 



Melhus (1911) found that direct germination may also occur, 

 governed by temperature. Palm (1932) observed that sporangia 

 of A. portidacae, A. bliti, and A. spimilosa, on germination, may 

 commonly form germ tubes. 



Antheridia and oogonia of white rusts begin to appear when 

 the production of sporangia is ceasing. They arise within the 

 host tissues. De Bary first described the sexual orgrans of x\lbuCTO, 

 and Wager (1896) gave a correct account of the details of fer- 

 tilization. The oogonia are globular, multinucleate cells, and one 

 or more small sflobular antheridia arise near the oocronium. 

 Gradually the oogonial content becomes differentiated into the 

 tq^^ cell, or ooplasm, and a peripheral layer, the periplasm. The 

 details which accompany these changes are carefully delineated 

 in a series of papers by Stevens (1899, 1901). He found that 

 there are essentially three types of fertilization. In the first, 

 represented in A. bliti and A. portidacae, \\hich he records as the 

 most primitive, there are many functional nuclei, a large recep- 

 tive papilla and a small coenocentrum. In A. tragopogonis, rep- 

 resenting the second type, all but one male and one female 



