Cannon : Anatomy of Phoradendron villosum Nutt. 381 



der, and cambiform cells occur between the cylinder and the cor- 

 tex. The cortical haustoria do not begin secondary growth until 

 they cease to function as food absorbers and serve the parasite as 

 food conductors, and to some extent as food reservoirs. With 

 this change in function appears a change in structure, and the cor- 

 tical haustoria pass over into the aerial haustoria which will be 

 spoken of later. 



Before examining the structure of the haustoria of the mistletoe 

 further it will be best to briefly glance at the main characters of 

 the structure of the host branch (Roble oak, Qnerciis lobatd) on 

 which the parasite grows. 



The oak branch is made up of a central cylinder surrounded 

 by a cortex. The cylinder is composed of a relatively large num- 

 ber of fibers, of a small amount of wood parenchyma, of radiating 

 medullary rays, and intermingled with other wood, elements, espe- 

 cially in spring wood, are also large and small tracheae, or ducts. 

 The wood parenchyma and medullary rays are living cells and are 

 well supplied with food. The ducts are lifeless and serve for the 

 transport of food materials in solution, which have been taken by 

 the roots of the host plant. Most of the prosenchyma cells have 

 scarcely any lumen and are lifeless, and it may be said in passing 

 that the oak owes its strength to the relatively great amount of 

 wood fibers of the wood cylinder. As indicated above, the wood 

 fibers of the mistletoe are present in relatively small numbers, and 

 in this connection a comparison of the two sorts of stems is of in- 

 terest. 



The cortex of the oak is made up of small parenchymatous 

 cells which are compactly set together, of several concentric rings 

 of sclerenchymatous fibers arranged in groups, and of scattered 

 grit cells. Instead of an epidermis, as in the mistletoe, the oak has 

 a few layers of cork which are derived from a superficially placed 

 cork cambium. A meristematic zone of cells, the cambium, sepa- 

 rates the cortex from the central cylinder. The outer fibers are 

 lignified, but the inner are not uniformly so. The grit cells are 

 also lignified. Of these tissues, the parenchyma and the meriste- 

 matic region are richly supplied with food, and it is among the 

 cells of these that the cortical haustoria of the parasite ramify. 



Taking up now the growth of the haustoria in the cortex of 



