PARASITISM AND RESISTANCE 381 



structures, but the exact mechanism of the transfer of food is not so well 

 understood. The haustorium of the filamentous parasite is very similar 

 in its behavior to the intracellular nonfilamentous parasite, being sur- 

 rounded by the protoplasm of the host cell. Haustoria may be of several 

 forms, simple and nearly spherical, coiled, and branched in various ways. 

 Most cytologists agree that there is a cellulose wall, or sheath, around the 

 older haustoria. It is presumably formed by the host cell and suggests 

 a weak mechanism of defense against the invading parasite, yet it does 

 not prevent the diffusion of soluble food into the haustorium. 



The haustorium commonly comes into contact with the nucleus of the 

 host cell. In 23 of the 35 cases (host-parasite combinations) reported 

 (Rice, 1927, 1935), habitual contact was observed between haustorium 

 and nucleus. Two theories as to the meaning of this contact have been 

 suggested. One is that the haustorium seeks out the region of the cell 

 nucleus in order to facilitate the absorption of food from the cell. The 

 second theory is that the action of the cell nucleus is defensive and that 

 in some cases it may cause the death and degeneration of the haustorium. 



In the case of Synchytrium (Chrysophylyctis) endohioticum the swarm 

 cells migrate into close proximity w^th the nucleus of the host cell (Orton 

 and Kern, 1919). In the majority of cases the nucleus is engulfed at the 

 time or soon after the swarm cells unite to form the vegetative body of the 

 parasite. The host nucleus disappears as the sporangia develop. The 

 exact significance of this close relationship between parasite and host 

 nucleus is not clear, but it apparently represents a more or less unique 

 method of parasitism among the fungi. 



It is generally believed that the balanced parasite causes harm to a 

 susceptible host primarily through its demand upon food and water. 

 There is little or no evidence that the protoplast is attacked chemically, 

 although host cells may be killed by growth pressure. There are numer- 

 ous reports of the disappearance of food in the region of haustoria. Butler 

 (1918) reported that starch is absent in cells containing haustoria of 

 Sclerospora graminicola, and at the time of sporulation the host cells 

 collapse and die. The only abnormal effect observed by Mains (1917) 

 on the cells of corn parasitized by Puccinia sorghi was the absence of 

 starch in the bundle sheaths near the rust pustules. He interpreted this 

 to mean that the parasite uses the food materials before they reach the 

 bundle sheath where they are normally stored. Similar disappearance 

 of starch in the host cells near infection by Synchytrium endohioticum has 

 been reported by Orton and Kern (1919). 



On the other hand, starch may accumulate in the infected tissues dur- 

 ing early stages of development of rusts but usually disappears in later 

 stages of development. This may be due to some disruption of the host's 

 physiology. The physiological reactions of the host are known to involve 



