Cannon: Anatomy of Phoradendron villosum Nutt. 379 



the stem, and will scarcely affect a similar one from which the 

 cuticle has not been removed. Further, if the fracture of the 

 mistletoe stem be compared with that of an oak a difference char- 

 acteristic of the structure of the two may be seen. The mistletoe 

 stem breaks straight across with no tendency to split, while the 

 oak breaks irregularly and splits at frequent intervals. 



3. The Haustoria 



The mistletoe is attached to its host by haustoria, which in 

 structure and in function show evident differentiation. The young 

 haustoria ramify in all directions in the cortex of the host and they 

 may even quite encircle the central cylinder.* From these others 

 turn sharply toward the center of the host branch, and become 

 partly enclosed by it. These latter are the so-called sinkers of the 

 parasite. Upon a considerable increase in diameter of the cortical 

 haustoria, the cells of the cortex of the host, which are outside 

 of the haustoria, gradually loose their contents and die and the 

 haustoria are in this manner exposed to the air- They undergo 

 considerable change in structure in the meantime. There are thus 

 three easily distinguishable sorts of haustoria, which, for conven- 

 ience I shall refer to as cortical haustoria, sinkers, and aerial haus- 

 toria. 



The youngest cortical haustoria are composed throughout of 

 thin-walled parenchyma. All of the cells are densely filled with 

 a coarsely granular material which gives a starch reaction. While 

 the cells of the interior of the haustoria are pretty much alike, those 

 corresponding to the epidermis are somewhat differentiated. In 

 some portions of the haustoria the outer cells appear much like 

 typical epidermal cells, and in others, at the growing parts, they 

 are much elongated and are papilla-like (Fig. 9). The latter recall 

 those in the region of the young haustoria in the dodder, f as well 

 as the epithelial cells in the scutellum of grasses, f and they appear 

 to have a function analogous to both. The elongated cells of the 

 mistletoe haustoria secrete a solvent that is capable of digesting 



*In Viscum the haustoria do not surround the stem of the host Kerner and 

 Oliver, Natural History of Plants, 208. 



t Peirce, Annals of Botany, Sept., 1893. 



rmer 



Munich, 1 888. 



