THALLOPHYTA: FUNGI 



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the Cruciferae, such as radish, turnip, mustard, and shepherd's-purse, 

 causing a disease known as white rust of crucifers. The rayceHum, which 

 may hve in almost any part of the host, ramifies throughout the inter- 

 cellular spaces and sends short button-Uke haustoria into the living cells. 

 Here and there beneath the epidermis the mycelium gives rise to com- 

 pact clusters of erect sporangiophores from the ends of which thin-walled, 



A C 



Fig. 90. Sporangia and sex organs of Albugo. A, cross section of a small portion of the 

 stem of shepherd's-purse, showing sporangiophores of Albngo Candida arising beneath the 

 epidermis and giving rise to multinucleate sporangia, X 600; B, sex organs of Albugo Candida, 

 a fertilizing tube from an antheridium penetrating an oogonium with a single nucleus in the 

 ooplasm, X750; C, oogonium of Albugo portulacae with multinucleate ooplasm, X500. 



globular, multinucleate sporangia are cut off in chains (Fig. 90.4). 

 These push up the epidermis and form a white blister on the surface of 

 the host. These blisters may appear on the leaves, stems, floral parts, 

 or fruits. Finally, the epidermis is ruptured and the sporangia are 

 carried by the wind to uninfected hosts. Here they give rise to 12 or 

 more laterally biciliate zoospores that escape, swim about for a while, 

 encyst, and finally produce new mycelia. When a spore germinates, it 

 produces a hypha that enters the host through a stoma. 



The sex organs of Albugo Candida, appearing later in the season than 

 the sporangia, are formed on the mycelium deep within the host tissues. 



