352 Eeport of the Mycologist of the 



diseases, the cause of which is not known, the blame had been 

 laid upon the weather. Fortunately the compound microscope 

 comes to our aid here and makes the whole matter perfectly plain. 

 If a fragment of leaf taken from one of the yellow spots is mag- 

 nified about 390 diameters, there will be seen a large number 

 of such things as are figured in Plate XII. These constitute the 

 downy mildew fungus, Plasmopara ciihensis, which is the real 

 cause of the yellow spots. It is not an insect. It is a vegetable 

 growth and is just as truly a plant as is the cucumber plant itself. 

 At Fig. 1 there is shown a branched sporophore (s) bearing sev- 

 eral young spores (sp). The sporophores are nearly colorless and 

 come out through the stomata on the under surface of the leaf, the 

 branched tops hanging downward. Fig. 3 shows one young and 

 one mature sporophore* coming through a stoma {st). When the 

 spores are mature they are violet colored and usually have the 

 form shown in Figs. 2, 2' and 2". These spores are readily carried 

 by the wind for a long distance. Should one chance to fall upon 

 a cucumiber leaf and find there a drop of dew or other moisture, 

 it will germinate in a few hours by discharging several small 

 protoplasmic bodies called zoospores. Each of the zoospores may 

 put out a germ-tube which finds it way through a stoma to the 

 Interior of the leaf, where it forms a net-work of colorless fungus 

 threads (hyphae) which run here and there among the cells. At 

 frequent intervals the hypae put out knob-like outgrowths which 

 penetrate into the cells and feed upon the cell contents. See Fig. 

 7, Plate XII. The fungus is, therefore, a parasite, appropriating 

 to its own use the nourishment which the cucumber plant has 

 prepared. Besides abstracting nourishment, the fungus probably 

 does further injury by poisoning the cells and causing them 

 to die quickly. After the fungus has vegetated within the leaf 

 for a while it forms sporophores which push out through the 

 stomata and produce another crop of spores. The length of time 

 required for the completion of this life cycle is not known, but it 

 is certainly short, prohably less than twenty-four hours. Thus 



♦The Bumber of sporophores which proceed from a single stoma is small, usually 

 one or two; but it is not uncommon to find as many as five, and even larger numbers 

 are occasionally seen. 



