254 HOST PENETRATION 



cells. Transfer of food from host to parasite does not require, in 

 this species, the production of specialized organs of penetration. 



A third type of ectoparasitism is exhibited by the Meliolaceae 

 and Capnodiaceae, which apply themselves to the surface of plant 

 tissues by means of hyphopodia. Graff (1932) found that, al- 

 though some species of Meliola form haustorial vesicles within the 

 epidermal cells, M. civ ductus is entirely superficial. Its cell walls 

 are in intimate contact with the host epidermis and are thinner 

 wherever contact is maintained. The epidermal cell walls are 

 more or less corroded at these points of contact, and evidence of 

 defeneration products was noted within them. 



Internal mycelium is wanting or scanty in many Microthy- 

 riaceae and Hemisphaeriaceae. Luttrell (1940) concluded that 

 Morejioclla quercina, one of the Microthyriaceae, absorbs its nu- 

 trients through the intact host cuticle at first, and later certain 

 hyphae penetrate only to the extent of becoming subcuticular. 



IMPLICATIONS 



Problems of host penetration remain of utmost importance in 

 spite of the large number of studies that have been devoted to this 

 phenomenon and in spite of the conflict among observations and 

 the interpretations of them. They should continue to receive un- 

 stinted attention because of their bearing on matters of tolerance 

 or resistance to disease, on studies involving the causes of natural 

 immunity, and on production of races of disease-resistant crop 

 plants. 



LITERATURE CITED 



Vi.i.en, Ruth F., "A cvtolomcal study of infection of Baart and Kanrcd 

 wheats bv Puccinia graminis tritici," J. Agr. Research, 23: 131-151, 1925. 

 "Cvtological studies of forms IX, XXI, XXVII of Puccinia grarmnis tritici 

 on Khalpi emmer," /. Agr. Research, 52:701-725, 1926. 



Arones< u, Ai ice, "Diplocarpon rosae: from spore germination to haustorium 

 formation," Bull. Torrey Botan. Club, 57:291-329, 1934. 



Barn. A. i>i . "(her einige Sclerotinicn und Sclerotienkrankheitcn," Botan. 

 Z., 44: 377-387, 393-404, 409-126, 433-441, 449-461, 465-474, 1886. 



Blackman, V. H., "Physiological aspects of parasitism," Proc. Brit. Assoc. 

 Bot. Toronto, 23 3-246, 1924. 



Bi..\( KMAN, y. H., and I". J. Wl lsford, "Studies in the physiology of para- 

 sitism. II. Infection by Botrytis ciuerea" Ann. Botany, 50:389-398, 

 1916. 



