378 Cannon : Anatomy of Phoradendron villosum Nutt. 



to do so when the change in composition of the walls takes place. 

 It should be said that the slime canals in Arceuthobium are of quite 

 different character, and also that they entirely, or for the most part, 

 occur in the cortex of the stem. Short cells with heavy, pitted 

 walls form the wood parenchyma and constitute a relatively large 

 proportion of the wood elements. Attention may be called here 

 to the structure of the wood cylinder as regards mechanical 

 strength although that will be considered more fully below. The 

 element of strength, the relatively large number of fibers, of the 

 oak stem, is seen to be wanting in the mistletoe, and those kinds 

 of cells which do not contribute primarily to the strength of the 

 stem, the wood parenchyma, and which are present in a small 



* 



amount in the oak, are again the most abundant in the mistletoe. 



The pith of the young stem is composed of cells which are quite 

 like those of the medullary rays and of the cortex, but in the old 

 stem they become heavily walled, and because of the presence of 

 pits, have a peculiarly ragged appearance in cross section. And 

 it should be said that perhaps the most striking character of the 

 stem is the presence in the pith and cortex, mainly in the pith, of 

 heavy walls and of correspondingly deep pits. 



The stem of this mistletoe, like the leaf, is mechanically very 

 weak. A comparatively small amount of force is needed to snap 

 it in two, and this weakness is in a measure independent of the size 

 of the stem. The reason for the brittleness can doubtless be found 

 in its structure. Among the factors that make for this condition 

 may be counted the great number of crystal -bearing cells, the 

 large number of grit cells, the presence of prominent intercellular 

 spaces, and of deep pits in the walls of the parenchymatous cells. 

 And to these may be added also the absence of secondary bast 

 fibers, the small number of wood fibers present, the peculiar tra- 

 cheid character of the wood ducts, and the relatively large amount 

 of xylem parenchyma. The greatest single source of strength is 

 probably the outer wall, the cuticle, of the periderm. If the cuti- 

 cle is carefully removed for a short distance, say 5 cm., from a 

 portion of the stem, the end of which is firmly secured, and 

 weights are placed on the free end, the stem will bend and finally 

 break in the region from which this has been removed. The in- 

 influence of the weights will hardly be observed in other parts of 



