LABORATORY STUDIES 225 



(i) In early summer examine the choke-cherry and plum 

 trees (wild and cultivated) for the young stages of Black Knot. 

 Watch the development until the knot becomes velvety in 

 appearance (about midsummer). Now make very thin cross- 

 sections of the knot and examine for conidia. The several 

 stages may be readily preserved in alcohol for future study. 



(j) Late in autumn and in early winter examine the knots on 

 the same trees. Note the young perithecia, i.e. hollow papillae. 

 Make very thin vertical sections through some of these. No 

 perfect ascospores can be found at this time. 



(k) Collect fresh knots in midwinter and make similar 

 examinations, when the asci and ascospores may be found. 



(/) In the autumn collect a quantity of leaves of the lilac 

 which are covered with a whitish mold-like growth, the Lilac- 

 mildew (Microsphaera alni). Scrape off a bit of this Mildew 

 after moistening with a drop of alcohol; mount carefully, 

 adding a little potassium hydrate. Look for conidia and 

 haustoria. Look also for spore-fruits, which appear like minute 

 dark dots to the naked eye. Carefully crush the spore-fruits 

 and observe the asci (four to seven) with their contained 

 ascospores (6). Note the beautifully branched tips of the 

 appendages. 



(m) Collect and study the mildews to be found on hops 

 (Sphaerotheca castagnei), on cherry- and apple-leaves (Podo- 

 sphaera oxyacanthae) , on hazel- and ironwood-leaves (Phyl- 

 lactinia suffulta), on willow-leaves (Uncinula salicis), on leaves 

 and fruit of grapes (U. necator), on wild sunflowers, verbenas, 

 etc. (Erysiphe cichoracearum) , on peas, grass, anemones, 

 buttercups, etc. (E. communis). 



(n) Place a few slips of green twigs in an ordinary plant-press, 

 allowing them to remain until they become (1) moldy (conidial 

 state), and (2) covered with minute yellow globular bodies (the 

 spore-fruits) . These are known as the Herbarium-mold ( A sper- 

 gillus herbariorurn) . Study as in the case of the Mildews. 

 This can frequently be obtained by placing a piece of almost 

 dry bread under a bell jar for a few days. 



(o) Blue Mold may be obtained from decaying fruit, pas- 

 try, etc. 



(p) Place a minute piece of "compressed" yeast upon a glass 

 slide, add a little water, cover with a cover-glass, tapping it 

 down gently. After a short examination under a high power of 



