9 6 



THE ANATOMY OF WOODY PLANTS 





elements are absent in the dicotyledons. No case is at present 

 known of their default in monocotyledons. There are two main 

 modifications of vascular elements or tracheae in the angiosperms 

 namely, vessels with scalariform perforations and those with 

 porous perforations. The term perforation is 

 applied to the actual apertures which make 

 their appearance in the terminal or other walls 

 of typical vessels. The first-named condition 

 of perforation is characteristic of lower groups 

 and lower genera among the dicotyledons, while 

 the second is usually found in the case of the 

 higher orders (the Compositae, for example) of 

 the dicotyledons and seems to be universal for 

 the monocotyledons. 



In Fig. 74 is represented a vessel of the 

 lower type that is, one in which the perfora- 

 tions in the terminal wall are scalariform. This 

 is an element from the root of the alder. The 

 lateral confines of the vessel are covered with 

 small bordered pits which in this particular case 

 are arranged in a somewhat regularly banded 

 fashion as a result of the contact of the con- 

 ducting element under discussion with the cells 

 of a ray of the wood. If the relation had been 

 with tracheids or other vessels, the arrangement 

 of the pores would not be so regular. Pits are 

 clearly seen in profile on the sides of the vessels, 

 indicating the fact that water-conducting ele- 

 ments belonging to this category do not neces- 

 sarily have their lateral pitting confined to 

 the radial walls. The ends of the vessel are 

 strongly inclined and are manifestly at definite 

 angles to the lateral walls. The illustration 

 presents the vascular element from the radial view; hence it is 

 obvious in this case that the perforated end walls are radial. This 

 is a situation seldom departed from in the w r oods of dicotyledonous 

 trees. In other words, a radial section of a dicotyledonous wood 



FIG. 74. Vessel 

 from Alnus, showing 

 the scalariform type 

 of perforation. 



