Cannon : Anatomy of Phoradendron villosum Nutt. 377 



chyma, and pith, and also, a cortex with a well-defined epidermis. 

 The young epidermal cells are cubical in form, but as the stem 

 increases in diameter they and the underlying cells for some dis- 

 tance become much flattened. The lateral and internal walls of the 

 epidermal cells bear numerous pits (Fig. 6). Some of the outer 

 cells project as trichomes and some are modified to form stomata 

 which are deeply sunken in the older parts of the stem (Fig. 5), 

 and are of the xerophytic type, the stomata of which are placed 

 longitudinally. 



The cortex of the young stem is composed of thin-walled 

 cells with intercellular spaces. As the stem becomes larger the 

 walls of these cells increase in thickness until in the older parts 

 they are relatively very heavy. They, at the same time, develop 

 numerous pits. The intercellular spaces also in the older stem 

 take on the characters which we have seen to be so pronounced 

 in the leaves. Groups of grit cells and cells bearing calcium oxa- 

 late crystals are noticeable characteristics of the old stems. 



Within the cortex of the young stem is a ring of several fibro- 

 vascular bundles, of the open collateral type, separated by medul- 

 lary rays. A group of hard bast fibers, which are derived from 

 the pro-phloem, is found inside of the pericycle exterior to each 

 bundle. Subsequently no hard bast is formed in the stem. Also, 

 on the inside of each bundle opposite the group of hard bast 

 fibers, is another group of hard bast-like fibers. These two 

 groups of sclerenchymatous tissue can be recognized in the old 

 stems although in these they occupy but a small proportion of 

 the entire area. The first definitive wood elements to be recog- 

 nized are the ringed tracheids, and later reticulated tracheids, 

 which with the growth of the stem become changed into tracheid- 

 like ducts by the reabsorption of the division walls. But the form 

 of the tracheids of which the ducts are composed is clearly recog- 

 nizable. Associated with the ducts are elongated cells, which 

 become thick-walled and form the wood fibers of the xylem. 

 When these fibers are young they present much the appearance 

 of slime canals. The contents of the fibers give the sugar reaction 

 with Fehling's solution, and in the younger ones, the walls are 

 cellulose. In all old fibers, however, these walls are lignified. If 

 the fibers function as slime canals at any time they probably cease 



