98 



BOTANY: PRINCIPLES AND PROBLEMS 



in their lack of a nucleus. They are elongated parallel to the 

 main axis of the stem and their end walls (more rarely their 

 sides) are provided with sieve-plates or definite groups of small 

 perforations. Through these perforations extend threads of 



Cambium 



IVood 



Bast 





Companion 

 Cell 



.Bast 

 Parenchyma 



Sieve 

 Cambium Fibers Tubes 



Fig. 50. — Radial longitudinal section of wood and bast of the tulip-tree 

 {Liriodcndron) . Section cut through the region shown in Fig. 49. At the left 

 is the wood, and at the right, the bast, with the canilMum between. The ladder- 

 like markings in the vessels are the end walls of the vessel-cells, and the small, 

 elliptical markings are pits in the side walls. The ends of the sieve-tubes are 

 occupied l)y sieve-plates. 



Vessels 



Bast 

 Fibers 



cytoplasm from one cell to another, so that the living substance 

 of each sieve-tube is directly continuous with that of the adjacent 

 ones. In the highest seed plants there is next to each sieve-tube 

 a small companion cell, provided with an abundance of cytoplasm 

 and a nucleus. In addition to these two types, groups of long 

 and very thick-walled cells, the hast-fibers, characteristically 



