388 Cannon : Anatomy of Phoradendron villosum Nutt. 



uniformly that those parts of the parasite which are the reservoirs 

 of food contained apparently the usual amount, and that the adja- 

 cent oak tissues did not show in any way that food passed to 

 them in the manner described. When the structure of the parasite 

 and its relation to the oak are considered it will appear that there is 

 really little opportunity for such a transfer of foods. The parts of 

 the mistletoe that contain food are either in contact with the life- 

 less host cells (the ducts of the wood), or they are adjacent to 

 host cells which, at the time, are dormant. 



The usual conception of the mistletoe as a semi-parasite, or a 

 water parasite, is for the most part a perfectly correct one. But 

 the degree of parasitism varies with the age of the plant, and with 



* 



the portion of the plant under consideration, for although the main 

 part of the parasite lives on the water and the mineral salts in 

 solution which it gets from the conductive system of the host and 

 the carbon that it gets from the air, a small portion of the cortical 

 haustoria depends upon the host entirely for its food and is there- 

 fore totally parasitic. The first assertion, that the degree of para- 

 sitism varies with the age of the plant, remains to be demonstrated, 

 but there seems little question that the seedling is a total parasite, 

 just as the cortical haustoria are. In this connection it may be of 

 interest to point out how such a total parasite as Arceitthobium 

 may have arisen. Arceuthobium lives in the cortex of its host, it 

 gets food already elaborated, and its only appearance in the air is 

 when it sends out the fruiting stalks. Now, if the American 

 mistletoe should acquire the habit of remaining a longer and 

 longer time after germinating in the cortex of the host, either 

 because of difficulty in penetrating the xylem of its host, or for 

 any other reason, it might gradually lose its power of carbon 

 assimilation and become a total parasite like Arceiithobiitm. 



The immediate effect of the parasite was not studied, but the 

 ultimate effect, in all cases that came to my notice, is to cause the 

 death of that part of the host which is beyond. There does not, 

 however, appear to be any such striking malformation as in the 

 Arceuthobimn, or in another species of Phoradendron that grows on 

 the juniper {P. bolleannm). Viscuvi also causes the decay of the 

 host tissues and in this resembles Phoradendron villosum, but the 

 effect is brought about in quite a different way. In Viscum the 



