CHAPTER III 



TRACHEIDS 



In the Gymnosperms the woody structure of the stem is com- 

 posed of more or less fibrous elements to which the generic 

 term tracheids may be applied in conformity with the definition 

 adopted by De Bary (13, 155), but which, for our present pur- 

 poses, may be described as elements of indeterminate or more 

 generally of determinate length, of either a prosenchymatous or 

 parenchymatous type, and characterized chiefly by the presence 

 of bordered pits. As such tracheids exhibit important varia- 

 tions among themselves, chiefly with respect to form, distribu- 

 tion, and structure, it is necessary to distinguish carefully between 

 the various types entering into the structure of the woody axis. 

 First of all it is possible to distinguish between those of the 

 prosenchymatous type and those of the parenchymatous type, 

 the differentiation being readily effected by means of the ex- 

 ternal form. Those of the parenchymatous type are to be met 

 with in either the medullary rays, when they may be described 

 as ray tracheids, or they arise in series parallel with the prosen- 

 chymatous elements with which they are therefore mingled as 

 wood parenchyma, and they are to be distinguished as paren- 

 chyma tracheids. A further discussion of these forms will be 

 deferred until they can be brought into connection with the gen- 

 eral structures of which they form characteristic features, while 

 at this time our attention may be directed more particularly 

 to the tracheids of the prosenchymatous type. The fibrous tra- 

 cheids are of two kinds, spiral and pitted. The spiral tracheids 

 are chiefly met with in the protoxylem region of which they are 

 characteristic and dominant elements and where they are of in- 

 determinate length. Their spirals represent a secondary growth 

 of the cell wall, and the latter is therefore devoid of pits 



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