380 Cannon: Anatomy of Phoradendron villosum Nutt. 



most, although it would seem not all, of the cortical tissues of the 

 host. These cells contain finely granular protoplasm, and nuclei 

 which are larger than those in the interior of the haustoria. The 

 host cells which are adjacent to those of the haustoria are rich in 

 food material and they stimulate to great activity the epithelial 

 haustorial cells just described. The adjacent host cells show evi- 

 dent signs of breaking down, and usually a small space intervenes 

 between them and those of the parasite. It has already been 

 stated that the epithelial cells appear to be unable to dissolve 

 certain of the host tissues ; these are the sclerenchyma of the 

 oak. The form and size of the digestive cells vary with the 

 changes in structure of the host cells. The former are relatively 

 long when the host tissues furnish them with an abundance of food, 

 but they become short and cubical when the adjacent host cells 

 are poor in food as, for instance, when they are near the surface of 

 the host stem. In the latter case the epithelial cells insensibly 

 merge into the more typical epidermal cells. Not only is the 

 shape and size of the epithelial cells dependent on the character 

 of the adjacent host tissue, but it will soon appear that the rami- 

 fications of the haustoria through the cortex are also dependent on 

 the same factor, because the epithelial cells make clear the way 

 for the advancing haustoria. 



The tissues within the epidermis of the cortical haustoria be- 

 come differentiated early into a central conductive system and a 

 surrounding cortex. The cortex is composed of thin-walled cells 

 with intercellular spaces. These cells, like those of the cortical 

 haustoria in general, except the conductive system and the epithe- 

 lium, are well filled with coarse grains of starch. Grit cells are 

 present in the cortex, but it is not possible in every case to say 

 whether they originated in the cortex of the host or that of the 

 parasite. Sections of young cortical haustoria frequently show 

 numerous masses of fully -developed grit cells in the midst of young 

 and thin-walled parenchyma, and such grit cells have been sur- 

 rounded, but not digested by the epithelial haustoria cells and come 

 finally to lie wholly within the tissues of the parasite. 



The conductive tissues of the cortical haustoria are developed 

 late and possess elements which resemble analogous portions of 

 the stem. Ducts appear in what may be termed the central cylin- 



