FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS 25 



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pits which are bordered on the side of the tracheids and are simple 

 on the side of the elements of the rays. 



In order to form an adequate conception of the nature of the 

 fibrous or tracheary elements of coniferous woods it is necessary 

 to view them in isolation from the tissues of which they form so 

 essential a part. It will be advanta- 

 geous to consider a simpler condition 

 first and then to proceed to the more 

 complex situation presented by the 

 elongated elements of the wood of 

 the pine. In Fig. 20 are shown tra- 

 cheids of the Big Tree (Sequoia 

 gigantea) belonging to the spring and 

 summer growth respectively. On the 

 left, one of the spring tracheids is 

 seen from its radial face. The cell is 

 obviously bluntly tapering at the ends 

 and has a length many times that of 

 its diameter. The pits which orna- 

 ment the radial aspect are of two 

 sizes. The larger bordered pits are 

 those which connect tracheid with 

 tracheid. The smaller pores brirrg 

 about relations between the rays and 

 the tracheids. The latter are bor- 

 dered only on the tracheary side, 

 although this feature is naturally not 

 obvious in the illustration. The tra- 

 cheid is represented as still surrounded 

 by its cement substance, which is indi- 

 cated by a heavier line. On the 

 lateral or tangential walls of the cell may be seen the profile aspect 

 of other bordered pits. These are tangential pits and are very 

 rarely present in the spring wood of conifers. Fig. 206 reproduces 

 the appearance of the same tracheid from the tangential side. 

 The general configuration of the fibrous element is now much more 

 pointed and only a few pits can be seen in face. On the side 



V 



FIG. 20. Tracheids of the 

 Big Tree (Sequoia gigantea) . Ex- 

 planation in the text. 



