44 



THE ANATOMY OF WOODY PLANTS 



tangential walls, a feature commonly found in the case of fibrous 

 elements in the terminal region of the annual ring in the conifers and 

 allied groups. The septate tracheid is 

 the most striking feature of the diagram. 

 In this element numerous transverse walls 

 interrupt the continuity of the central 

 lumen. Certain of these transverse 

 partitions are characterized by the pres- 

 ence of bordered pits, while others offer to 

 the eye pits of the simple type. In a 

 third condition bordered and simple pits 

 confront one another in the same wall. 

 Those cells which communicate with the 

 surrounding elements by means of simple 

 pits are typically occupied by protoplas- 

 mic contents, while the elements possess- 

 ing pores of the bordered category are 

 invariably without living substance. It 

 will be clear to the reader that, if the 

 diagrammatic representation presented in 

 Fig. 34 is correct, the parenchyma tous 

 elements of the wood come from the sub- 

 division of the primordial elements which 

 would in other cases and under different 

 circumstances give rise to ordinary tra- 

 cheids. At an early stage these elements 

 became transversely septate, and in the 

 segments so set off the protoplasm some- 

 times persists (when a typical parenchy- 

 matous element of the wood is the result) ; 

 at other times it disappears with the 

 complete differentiation of the walls 

 surrounding it (hi the case of so-called 

 short tracheids). An interesting fact in 

 this connection is the occurrence of paren- 

 chymatous storage elements in the same 

 region of the wood where the tangential 

 pits take their origin. It has been made 



FIG. 34. Diagrammatic 

 representation of the origin 

 of wood parenchyma in the 

 Picea. 



