ALBUGO 109 



3. Watch for the escape of zoospores, which almost always takes place 

 from mature sporangia, when mounted on the slide, probably because 

 of the somewhat changed conditions of temperature and density of the 

 water. Observe whether the zoospores swim away at once (generally 

 Saprolegnia) or immediately come to rest near the opening of the 

 sporangium (generally Achlya). 



4. Stain the zoospores with iodine to show cilia. 



C. Watch for the development of oogonia as clusters of minute spher- 

 ical structures, generally situated nearer the substratum than the 

 sporangia. Observe : 



1. That the oogonium develops a number of eggs. Count them. Draw. 



2. Whether delicate antheridial filaments are present, growing over the 

 surface of the oogonia and entering them. Draw, if present, and 

 study their origin. 



109. Albugo, the blister blight. This parasite is common on the shepherd's 

 purse (Capsella), which is the most convenient host for laboratory study. 

 Note the appearance of white blisters, the conidial fructification, on leaves 

 and stems. The sexual fructification, more common on the radish, occurs 

 in the interior of stems and leaves, which become swollen and purplish in 

 color. Make a habit sketch of the blisters. 



A. Study sections of the blisters cut free-hand in pith or preparations cut 

 on the microtome (Sec. 212). Draw : 



1. An outline of the section showing (a) the position of the epidermis, 

 (6) the chains of air spores, or conidia, which raise the epidermis 

 from the tissue beneath, and (c) the position of the conidiophores, 

 structures from which the conidia develop. 



2. Under h.p. the details of a group of conidiophores, showing the 

 manner in which the conidia develop and the relation of the conidio- 

 phores to the mycelium in the interior of the host. 



3. Study the mycelium between the cells of the host. Search for 

 sucker-like processes, haustoria, penetrating the host cells. A portion 

 of infected tissue boiled for a few minutes in a dilute potash solution 

 (five per cent) and then teased out will give excellent preparations. 



B. Study sections of tissue with the sexual organs. Note and draw : 



1. The large oogonia, the oldest ones containing each a single oospore 

 with a heavy cell wall. 



2. Younger oogonia in which the egg is present as a region of denser 

 ooplasm, separated from a surrounding periplasm; also younger 

 stages before the egg is differentiated in the oogonium. 



3. Favorable sections, if present, showing the club-shaped antheridium 

 at the side of the oBgonium and the beak-like process which it puts 

 forth, penetrating the oogonium and growing through the periplasm 

 to the egg. 



