PERISPORIALES 



193 



rusts they belong to the few groups of obligate parasites which up to the 

 present have resisted all attempts at culture on artificial substrates; as 

 in the rusts, they do not kill the affected host tissue but may stimulate 

 it to slight hypertrophies. 



The hyphae are strongly septate; their cells are uninucleate; the 

 haustoria alone are sometimes multinucleate. Their walls are hyaline, 

 except in Sphaerotheca mors-uvae where they become dark in age. 



As regards their behavior toward the host, they may be arranged in a 

 noteworthy series from endoparasitic, through hemiendophytic, to ecto- 

 parasitic forms. Their lowest endoparasitic stage is represented by 



Fig. 120. — Erysiphe Polygoni on Geranium maculatum. Development of haustoria. 



( X 1,200; after G. Smith, 1900.) 



Leveillula taurica (Oidiopsis taurica). In it the mycelium lies, as in many 

 parasites, in the intercellular spaces of the host leaves; only in the later 

 stages of conidial formation do the hyphae emerge on the surface of the 

 leaf and then form, like the other Erysiphaceae, an arachnoid or felty 

 mat. These extramatrical hyphae cling by appressoria to the sur- 

 face of the leaf; on this surface, however, they form no haustoria but 

 nourish themselves, as do the hemiendophytic forms, by branches 

 which penetrate the interior of the leaf through the stomata. 



In the hemiendophytic stage, as in Phyllactinia, the mycelium lives 

 in the manner characteristic of mildews, extramatrically on the 



