Cannon : Anatomy of Phoradendron villosum Nutt. 385 



wholly exposed to the air, and have undergone marked changes in 

 structure and in function. As the cortical haustoria increase in 

 bulk and occupy a larger and larger area in the host cortex, the 

 flow of foods and food materials from the central to the more pe- 

 ripheral portions of the cortex is gradually checked, and the outer 

 cells of the host cortex die. The inner cells of the host cortex, 

 however, remain alive, and may form cork cambium between the 

 aerial haustoria and the xylem of the host. By the multiplication 

 of the cortex cells beneath the aerial haustoria, a considerable pres- 

 sure is brought to bear upon the haustoria, which is exerted in such 

 a way as to tend to push them away from the host branch. A 

 flattening beneath the haustoria of the host cortical cells in ques- 

 tion shows this point very plainly. 



Parallel to the decadence of the superficial host tissues a change 

 is noted in the structure of the cortical haustoria. This is first 

 seen in the epidermis. The outer walls of the epidermis become 

 greatly thickened and take on the bronze color peculiar to the ex- 

 posed portions of the plant This, especially the latter, applies to 

 the parts of the epidermis which are exposed to the air. In sec- 

 tions through the edge of the dying cortex and also through the 

 epidermis of the haustoria, the formation of the cuticle as well as 

 the deposition of the coloring matter in it, under the influence of the 

 light, may easily be seen. Except for the greater irregularity, the 

 absence of trichomes and of stomates, the epidermis of the aerial 

 haustoria agrees very well with that of the stem. 



The structure of the cortex of the aerial haustoria is also in 

 the main like that of the stem, but the cells of which it is com- 

 posed are more irregular in form and in size. Grit cells and crys- 

 tal-bearing cells are present in the cortex. 



Cross sections of the wood cylinder of the aerial haustoria 

 have a striking appearance, which is caused by the unsymmetrical 

 growth of its members, the secondary growth in diameter taking 

 place on one side, toward the outside of the stem of the host, 

 more than on the other. The elements of the central cylinder of 

 the haustoria resemble those of the stem, but the pith, the primary 

 vessels and the central ring of sclerenchyma are wanting. The 

 peculiar structure of many of the fibers, by which they resemble 

 slime canals, and which was spoken of as occurring in the stem, 



