CHAPTER IV 



THE FIBRO VASCULAR TISSUES: SECONDARY WOOD 

 TRACHEIDS AND FIBERS 



Very important constituents of all woody organizations are the 

 tracheids and fibers. These present a wide range of structure 

 from the lower forms to the higher and illustrate some interesting 

 general evolutionary principles. Fig. 19 is a highly magnified 

 transverse view of the wood of the white pine (Pinus strobus) in the 

 region of transition from one annual ring to the next. Woods of 



this type are very simple and consist 

 mostly of elongated tapering elements 

 with bordered pits in their walls and 

 known as tracheids. The only fea- 

 tures of organization not tracheary in 

 their nature are the rays and the 

 resin cavity. The tracheids are dis- 

 tinctly of two kinds. Some are large 

 and thinner-walled and begin the 

 annual ring as the so-called spring 

 elements. Others, thicker as to their 

 walls and with a smaller lumen or cen- 

 tral cavity, constitute the summer 

 tracheids. The two kinds of tra- 

 cheids are in further contrast with 

 one another because of the position 

 of the bordered pits on their walls. In the spring elements the 

 pits are confined to the radial walls that is, those sides of the 

 fibers which are either in actual contact with, or are parallel to, 

 rays. In the case of the summer tracheary cells pits are pre- 

 dominant on the tangential walls which are at right angles to 

 the rays. In addition to being in communication with one 

 another by means of bordered pits, the tracheids both of the 

 spring and of the summer wood are likewise related to the rays by 



24 



FIG. IQ. Part of a transverse 

 section of the wood of the white 

 pine (Pin us Strobus), showing 

 radial and tangential pitting of 

 the tracheids. 



