4 20 BOTANY OF THE LIVING PLANT 



leaf by haustoria, which penetrate the outer walls of the epidermal 

 cells, the connection being established immediately on the germination 

 of the infecting conidium (Fig. 317). As the season progresses 

 small dark specks make their appearance, large enough to be seen 

 with the naked eye. These are the fruits, or perithecia, each con- 

 taining one or more asci. In damp weather these parasites develop 

 quickly, a fact that has drawn almost superstitious attention to them. 

 A very common example is seen in Podosphaera clandestina, which 

 infests the leaves of the Hawthorn in any warm and wet autumn. 



Fig. 318. 



Piece of epidermis of Hop, showing mycelium (b) and perithecia (a) of the Hop- 

 mildew on its surface, h is a large hair. At (b) the first beginnings of a perithecium. 

 Magnified. (After Marshall Ward.) 



A still simpler one, Sphaerotheca Hamuli, causes a disease on the 

 cultivated Hop, though it may occur also on many other plants. 

 The effect of Mildews on the infected plant is that the diseased areas 

 take a pale colour, nourishment being withdrawn from them. But 

 there is no malformation. 



After the Hop Mildew has established itself, and formed a branched, 

 septate mycelium, certain hyphae grow vertically upwards from 

 the leaf-surface without branching : they segment transversely into 

 short lengths, which become detached in basipetal series as conidia, 

 easily removed when ripe by a breath of air. They will germinate 

 on a moist leaf, and cause new infections during the summer (Fig. 3 17)- 

 But later the fruit-bodies (perithecia) appear, which provide for 

 the winter's rest (Fig. 318). They arise where the branched hyphae 



