196 PH\XUM IV. SIPHOXOPHYCEAE 



with whitish masses of radiating hj'-phae of Saprolegnia or 

 related genera. Remove some of these hyphae and examine 

 for zoospore formation. Somewhat later oogones and antherids 

 may often be found. A water mold {Saprolegnia ferax) 

 frequently occurs upon the bodies of young fishes, especially in 

 fish-hatcheries where it is occasionally very destructive. 



(/) In the Spring the leaves and stems of shepherds'-purse 

 and peppergrass may often be found covered underneath with 

 a white mold-like growth {Peronospora parasitica). Carefully 

 scrape off a little of this growth and mount first in alcohol, 

 afterward adding a little potassium hydrate. The irregularly 

 branching filaments will be seen to bear here and there white, 

 broadly ellipsoidal conidia. Similar studies may be made of 

 the Grape-mildew {Plasmopara viticola) on grape-leaves in 

 autumn, and the Lettuce-mildew {Bremia lactucae) on cultivated 

 and wild lettuce from spring to autumn. 



(g) Make very thin cross-sections of a leaf affected with a 

 Downy Mildew, when the latter has passed the period of its 

 greatest vegetative activity. Mount in alcohol (to drive out 

 air-bubbles), then add potassium hj'drate, and look for the 

 resting-spores, which in some species are of a dark brown color. 



(h) White Rusts occur on man}?- plants: one {Albugo Candida) 

 on shepherd's-purse, peppergrass, radish, etc.; another {A. 

 hliti) on Amaranthus; and another (.4. portulacae) on purslane. 

 For conidia make very thin cross-sections of leaves, through a 

 white-rust spot, and mount as above. The resting spores 

 (which are dark brown) are easily obtained in the leaves of 

 Amaranthus and purslane and in the distorted stem of the 

 radish. 



{i) In the study of Black Molds it is mostly necessary to 

 make use of alcohol for freeing the specimens of air; afterward 

 they usually require to be treated with a dilute alkah (as a 

 weak solution of ammonia or potassium hydrate), which 

 causes the filaments to swell up to their original proportions. 



{j) Cut a lemon in two, and, squeezing out most of the juice, 

 expose the two halves to the air af an ordinary laboratory or 

 living-room for a few days, when various molds will begin to 

 develop. Under favorable circumstances Black Mold (Mucor) 

 will predominate. It can be told by its dark color and the 

 minute round black sporangia on the ends of the erect filaments. 



