CHAPTER VI 



MEDULLARY RAYS (continued} 

 RAY TRACHEIDS 



In the higher Coniferae the medullary ray is distinguished 

 by the presence of an element which differs materially in its 

 structure from the associated parenchyma cells. These elements 

 have been designated as ray tracheids (13, 491-492). Their struc- 

 ture is so peculiar, and they present such important relations 

 to classification and development that a somewhat detailed 

 account of them is necessary, to some extent in recapitulation of 

 well-known observations (13, 461 ; 21, 13 ; 82). 



As stated by De Bary, the ray tracheid resembles the paren- 

 chyma cells, from which they differ, however, in the presence 

 of bordered pits on all their walls. Furthermore such pits not 

 only differ materially in form and size from the bordered pits 

 of adjacent parenchyma cells, but they are always much smaller 

 than the pits of those wood tracheids on which they border. 

 Such tracheids are invariable features of the ray in all the higher 

 Coniferae from Tsuga and Pseudotsuga to Finns, to the extent 

 of 25 per cent of the investigated genera. In Juniperus they 

 occur very rarely, being found, so far as I am aware, in only one 

 species (J. nana) out of a total of eleven, and they are so sparingly 

 developed as to readily escape observation. In Thuya they are 

 to be met with in T. japonica, likewise in a rudimentary state of 

 development. Out of nine species of Cupressus they occur only 

 in C. nootkatensis. Of the ten investigated species of Abies they 

 are found only in A. balsamea. In commenting upon this fact 

 many years since (13, 490), it was also pointed out that among 

 European species A. excelsa is similarly exceptional, but no 

 attempt has been made to interpret the significance of such facts. 



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