FIBROVASCULAR TISSUES: PHLOEM 125 



either end. For comparison with this condition is shown in b the 

 sieve element of an arboreal dicotyledon. Here the terminal walls 

 are distinctly at angles with the lateral ones and bear the best- 

 developed sieve plates. Laterally, on two opposite and usually 

 tangential sides, are situated the degenerate sieve plates, or lattices. 

 The third item, c, visualizes the type of sieve tube characteristic 

 of an advanced herbaceous dicotyledon or of a monocotyledon. In 

 this instance the terminal wall of the sieve tube is horizontal, and 

 parietal lattices are conspicuous by their absence. This state 

 represents the highest degree of differentiation of the sieve element. 

 In the following items are delineated corresponding conditions for 

 the water-conducting elements of the xylem. In d appears a tra- 

 cheid from the spring wood of Pinus, showing tapering or fusiform 

 configuration and exclusively radial pitting. Next, in e, appear 

 the main vessel types of Gnetales and perennial dicotyledons, 

 showing inclined terminal walls which in one instance have scalari- 

 f orm perforation of the lower and in the other the porous perforation 

 of the higher perennial types. Finally, in/ is depicted the vessel of 

 the monocotyledons and extreme herbaceous dicotyledons. Here 

 the terminal walls are practically horizontal, and the perforation is 

 porous (in the case of the monocotyledons invariably so). 



