FIBROVASCULAR TISSUES: VESSELS 



103 



Where vessel comes in contact with vessel, we find frequently a 

 mode of pitting characteristic of a genus or even a family. Details 

 in this connection are beyond the range of a work so elementary 

 as the present one, but a few prominent features may be indicated. 

 As has been shown in the figure of the vessel of Liriodendron above 

 (Fig. 76) , lateral pits may be in horizontal rows. In other instances, 

 as in the oak shown hi Fig. Boa, the lateral pits are arranged in an 

 alternating fashion. Again, in b, representing the lateral pitting 

 of the vessel in the poplar, alternation is accompanied by crowding 

 to such an extent that the pits become angular by mutual contact. 



FIG. 80. Lateral pitting of vessels of dicotyledons. Explanation in the text 



In c is shown the characteristic pitting of the vessels of the vine 

 family (Vitaceae). Here the sculpture of the lateral walls consists 

 of elongated slits closed by equally elongated membranes. A 

 reference to the structure of the secondary wood in the region of 

 the pith in this instance makes it clear that the slitlike pits of the 

 Vitaceae are not persistent scalariform pores of the first-formed 

 region of the xylem, but are the result of the lateral fusion of 

 horizontal rows of pits in the side walls of the vessels. In d is 

 represented a condition of structure of the vascular wall which is 

 common in the higher types of dicotyledonous woods. Here the 

 alternating pitting is overlaid internally by spiral structures which 

 it is customary to call tertiary thickenings. 



In concluding the statements in regard to vessels it is necessary 

 to refer to the phenomenon of tylosis or occlusion of the lumina 

 or cavities of the vessels by parenchymatous ingrowths. These 



