ASCOMYCETES 



231 



On the willow the area occupied by the mycelium sometimes 

 shows a tendency to retain its chlorophyll longer than other por- 

 tions of the leaf. This stimulating effect of a parasite is, however, 

 best marked in the case of Uncinula Aceris (De C.) Wint., occur- 

 ring on several species of maple (Acer). The yellow leaves in the 

 late autumn may show definite green areas, which will be found 

 to be the parts of the leaf occupied by the fungus (Fig. 95). 



XXX. COMMON MILDEW OF TREES 

 Phyllactinia Corylea (Pers.) Karst. 



Palla, E. Ueber die Gattung Phyllactinia. Ber. d. deut. bot. Ges. 17 : 64- 



72. pi. j. 1899. 

 Salmon, E. S. On Certain Structures in Phyllactinia. Journ. Bot. 37: 449- 



454. pi. 402. 1899. 



This species of mildew is so distinct from those previously dis- 

 cussed that it is by some 

 made the type of a sub- 

 family. As previously 

 stated, no haustoria are 

 present, but special seta- 

 like branches penetrate 

 the host. The perithecium 

 is large and provided with 

 hyaline, rigid, acicular ap- 

 pendages, each with a swol- 

 len base. There are many 

 asci, containing 2 or 3 

 spores (Fig. 86, e). The de- 

 velopment of the asci has 

 been discussed (Fig. 85). 



This species occurs more 

 commonly upon shrubs or 

 trees, but it is also para- 

 sitic upon a limited num- 

 ber of herbaceous plants. 

 It is known to be distributed throughout the northern hemisphere, 

 and is frequently one of the more common of the surface mildews. 



Fig. 95. Yellow Leaf of Maple, with 

 Green Areas occupied by Uncinula 



