TISSUES AND SIMPLE ORGANS. 73 



tracheids closely resemble the vessels in structure, the fibrous or 

 long-celled tracheids represent a form intermediate between me- 

 chanical cells and tracheal elements. The fibrous tracheids are 

 therefore dead prosenchy ma-cells with numerous bordered pores. 

 They occur very abundantly in the wood of conifers ; they are 

 also frequently intermingled with the various tissue-elements of 

 angiospermous trees. The term " bordered -porous -libriform " 

 is sometimes used to designate these tracheids ; this term is in- 

 tended to imply that they resemble mechanical cells in structure 

 and function, but that they differ from specific mechanical cells 

 (bast-cells and libriform-cells of trees) in having no bordered pores, 

 but only such without borders. The absence of the primordial 

 utricle in the mature libriform is not a characteristic of this cell- 

 form, although it is frequently spoken of as a dead tissue, and this 

 with some degree of justification. Woody parenchyma and medul- 

 lary rays are physiologically equal in so far as both consist of living 

 cells which at definite periods carry considerable carbohydrate as 

 well as physiologically related substances ; anatomically they also 

 resemble those parenchymatous cells having numerous rounded 

 simple pores ; they differ, however, in their position and arrange- 

 ment. Woody parenchyma usually extends longitudinally in the 

 form of bundles or bands ; sometimes it slants in a tangential or 

 radial direction. Medullary rays represent radial bands or plates 

 having the form of cell-surfaces or expanded cell-complexes. The 

 individual cell of the wood- parenchyma is regularly elongated in 

 direction of the axis of growth ; the cell of the medullary ray is 

 elongated in a radial direction, at least very frequently. The func- 

 tion of conduction is not only assisted by this special arrangement 

 of the wood-parenchyma and medullary ray, but also by the numer- 

 ous pores occurring in the most suitable parts of the cell-walls. In 

 the medullary rays the pores are therefore most numerous in the 

 tangential walls. (Cell-forms intermediate between wood-paren- 

 chyma and libriform are named "substitute fibres" (Ersatzfaserri) 

 by SA^IO.) A more or less theoretical observation may be intro- 

 duced here, namely, that cells resembling wood-parenchyma also 

 occur outside of the cambium between the cortical medullary rays. 

 This might be a reason why the term " woody parenchyma " should 

 be rejected, since this cell-form occurs not only in the woody paren- 

 chyma, but also in the inner cortical tissue, as well as within and 

 also outside of the monocotyledonous vascular bundles. But because 



