CHAPTER II. — PLANT TISSUES. 



163 



lignified, their lumen commonly filled with air, though sometimes 

 with cell-sap, and they are destitute of protoplasm. The thick- 

 enings, which constitute the markings, are on the internal sur- 

 face of the wall, and the different kinds of ducts are named 

 from the character of these markings. 



The following kinds are the most important : 



a. The Dotted or pitted are characterized by rounded or 

 oblong, thin areas or pits scattered over the wall, as in Fig. 418. 



b. The Scalariform differs from the dotted chiefly in the 

 fact that the pits are greatly elongated in a transverse direction, 

 giving rise to markings which bear some 

 resemblance to the rounds and spaces of a 

 ladder; hence the name ; Fig. 419. 



a—\ 



a - 



Fig. 423. 



Fig. 423. -Transverse view of trabecular tracheids from 

 leaf of common Juniper: /, a bordered pit: a, thickening cross- 

 ing the lumen of the cell. Magnified ^80 diameters. 



Fig. 424. —Longitudinal view of the same, magnified to the 

 same extent. The letters also refer to the same structures. 



Fig. 424. 



c. The Spiral are those in which the markings consist of 

 spiral thickenings. The spirals may be loosely or compactly 

 arranged ; they may be single, double, treble, or even sextuple. 

 The rest of the wall on which they are borne is usually thin, so 

 thin that when a stem or other organ containing them is torn 

 asunder by a longitudinal strain, the cell-wall is ruptured, and 

 the spirals are drawn out sometimes to great length, and appear 

 to the naked eye like spider lines. Two of these ducts are 

 shown in Fig. 420, a and b. 



d. The Annular duct is one in which the thickenings take 

 the form of rings, Fig. 421. Transition forms between these and 



