Trees — The Character, Structure, Sec. 51 



ends knocked out, and usually having on their walls bordered 

 pits and reticulated thickenings. A vessel, then, is a cell fusion, 

 while a tracheid is a cell. They may range from the diameter of 

 a pin, as in Oak, down to those too small to lie seen with the 

 naked eye, as in Willow. Hardwoods on this account are some- 

 times called "porous woods," while softAvoods having no vessels 

 are frequently called non-porous woods." 



Among the hardwoods, those forming a prominent porous 

 ring are sometimes described as "Ring porous woods" such as 

 Oak, Chestnut, and Elm, while those examples where the pores 

 are more or less evenly distributed throughout the entire ring 

 are frequently described as ' Diffuse porous woods " such as Birch, 

 Hornbeam, Alder, Po[)lar, and Willow. The rings are best 

 defined in ring-porous woods and non-porous woods. (Fig. 2). 



Tyloses. — These are ingrowths of cells adjoining the pore 

 (vessel) of some hardwoods or a resin pore of some softwoods, or 

 more rarely in the tracheids of such hardwoods as sweet gum. 

 {Liqiiidamber styraciflua). These cells make their invasion by 

 pushing their way through the pits or unthickened portions of 

 the walls into the cavity. They contain protoplasm and in some 

 cases nuclei, and may divide and completely plug up the lumina 

 of the elements which they invade, forming partition walls at 

 irregular intervals throughout the entire length of the tube. 



Tyloses occur in many woods, sometimes few and sometimes 

 in abundance. We have examples in some Oaks, Sycamore, 

 Kobinia, and Snakewood {Brosimum aiihletti). In the last named 

 the tyloses become thickened and converted into stone cells. 

 Tyloses originate under various conditions, but are more usually 

 found in connection with the transition of the sapwood into 

 heartwood, especially in those woods with large pores, rendering 

 the heartwood impervious to water. They are also produced in 

 the pores of wood wherever local injury takes place, as in cut 

 branches, and they develop in the cementing tissue during the 

 progress of engrafting. 



SnoAv points out that air can be readily blown through 



