PENETRATION BY ECTOPARASITES 253 



ity to that necessary to maintain a delicate nutritional balance. 

 Haustoria mav be uninucleate, may contain a pair of nuclei, or 

 may be multinucleate. They possess a conspicuous sheath that is 

 deposited by and is continuous with the host-cell wall, as generally 

 believed. In Diplocarpon rosae the sheath does not extend com- 

 pletely around the haustorium [Aronescu (1934)1, as has been 

 described for many parasitic fungi. In Erysiphaceae, however, 

 staining reactions indicate that haustorial sheaths are chitinous. 

 More should be known regarding the chemical nature of the 

 sheath as an aid in understanding how the sheath modifies absorp- 

 tion and passage of food. 



Haustoria are always connected with the intercellular hyphae 

 by narrow tubes of a length slightly in excess of the thickness 

 of the host-cell membrane. These constrictions facilitate pene- 

 tration. Presumably both mechanical pressure and dissolution of 

 the wall are involved in haustorial penetration. Allen (1923) 

 found no evidence of enzyme secretion in connection with haus- 

 torial penetration by Puccinia graminla tritici, but the walls be- 

 neath the appressoria appeared to be altered during initiation of 

 infection. 



PENETRATION BY ECTOPARASITES 



Several distinct types of host-parasite relationships occur among 

 fungi possessing mycelium which remains wholly external to the 

 infected plant organs. One type is represented by the powdery 

 mildews, all of which, except Fhyllactinia cor y lea and Leveillula 

 (Erysiphe) taurica, are ectophytic. In P. cor y lea both internal 

 and external mycelium is produced; in L. taurica the mycelium is 

 wholly internal, an adaptation to xerophytic environment. All 

 parts of all other powdery mildews, except the haustoria, are 

 borne externally. 



A very unusual type of ectoparasitism is exhibited by Cerco- 

 sporella rubi, the cause of rosette and double blossom of blackber- 

 ries and dewberries. Plakidas (1937) found that the mycelium 

 of this fungus occurs in buds between the embryonic leaves. If 

 the buds are opened at any time during summer, fall, winter, or 

 early spring, a delicate arachnoid weft will be observed to be pres- 

 ent between the young leaves. The fungus at no time actually 

 penetrates the young branch buds and flower buds but absorbs 

 its nourishment directly through the thin walls of the embryonic 



