PYTHIUM. 137 



close contact with the oogonium : a cylindrical process 

 from it passes through the wall of the oogonium, an'd 

 gains access to the oosphere. 



c. In more mature specimens the oogonium contains 

 a single round, distinctly walled cell (the oospore), 

 which lies freely within it. 



In order to follow the process of fertilisation, which is to be 

 seen with distinctness in this plant, portions of a well-nourished 

 culture should be cultivated in a damp chamber, in a suspended 

 drop of water : by selecting an oogonium and antheridium of 

 suitable age, the actual transfer of the granular protoplasm from 

 the antheridium to the oosphere may be followed. (See Marshall 

 Ward, Q. J.M.S. xxiii., page 490, &c. ) 



Cystopus candidus (White Rust of Shepherd's 

 Purse). 



Observations on Cystopus are so easily made that they should 

 not be omitted. The Fungus is commonly to be found growing 

 parasitically upon Capsella Bursa-pastoris, and others of the 

 Cruciferce, causing in summer white eruptions, accompanied by 

 swelling, and greater or less malformation of the vegetative 

 organs, and even of the flower and fruit. It must not be con- 

 founded with the equally common Peronospora parasitica, which 

 is to be found at similar times on similar plants, and bears a 

 superficial resemblance to it. 



Note with a lens the white patches : some will be seen to be 

 still completely covered by the epidermis, in other cases rupture 

 will have taken place, and the mealy mass of white conidia is 

 thus exposed. 



Cut longitudinal sections of the stem of Capsella so as to 

 traverse one of the blotches, and, examining under a high power, 

 observe 



1. The general arrangement of tissues as in a typical 

 Dicotyledon. 



