ECONOMIC WOODS OF THE UNITED STATES 133 



the segment does not. Sometimes segments are fused through 

 their lateral walls, or the end of one segment may be joined to the 

 lateral wall of another, but such forms are to be considered ex- 

 ceptional. 



The plane of contact between segments may be : (a) horizontal 

 or transverse, that is, at right angles to the axis of the vessel; (6) 

 oblique or inclined, almost always facing the ray; (c) vertical or 

 longitudinal. The last may be considered an extreme form of the 

 oblique unless it occurs where segments are fused through their 

 lateral walls. 



VESSEL PERFORATIONS 



The opening from one segment into another is called the vessel 

 perforation. The various types and modifications of vessel per- 

 forations supply features highly important for diagnostic purposes. 

 The two principal forms are : (a) the simple and (6) the scalariform. 

 Insofar as our commercial woods are concerned knowledge of these 

 two types is sufficient. There are various other forms, however, 

 though most of them are modifications or malformations of the 

 two principal types. The reticulate form is not uncommon, 

 especially in the Rosaceae, and tendency to it is seen in the branch- 

 ing and anastomosing bars in almost all woods with scalariform 

 perforations. In Rosa sp. the author has observed in a single 

 section, simple (predominating), scalariform, reticulate, pit- 

 perforate, and various composite perforations. 



The following classification shows the range of variation in 

 perforations, though there are innumerable forms of the composite. 

 A few instances, mostly exotics, are cited as illustrations of the 

 rarer kinds. 



TYPES OF VESSEL PERFORATIONS 



SIMPLE : (Single opening, circular, elliptical or elongated-elliptical.) 

 SCALARIFORM: (Openings slit-like or elongated between cross 



bars.) 



Bars transverse (Common form). 

 Bars vertical (Very rare. In certain Compositse and Axyris 



amarantoides) . 



RETICULATE : (Irregular openings as meshes between anastomosing 

 bars.) 



