196 PHYLUM IV. SIPHONOPHYCEAE 
with whitish masses of radiating hyphae 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. 
(f) In the Spring the leaves and stems of shepherds’-purse 
and peppvergrass 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 hydrate, and look for the 
resting-spores, which in some species are of a dark brown color. 
(h) White Rusts occur on many plants: one (Albugo candida) 
on shepherd’s-purse, peppergrass, radish, etc.; another (A. 
bliti) on Amaranthus; and another (A. 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. 
(t) 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 alkali (as a 
weak solution of ammonia or potassium hydrate), which 
causes the filaments to swell up to their original proportions. 
(7) 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. 
