^^^^ ' ANATOMY 139 



With a narrower pore occupying an obliquely vertical position, 

 a form of pit well shown in some species of Podocarpus. In the 

 Podocarpineae and in most of the Cupressineae the tangential 

 and horizontal walls of the ray cells are unpitted. An examination ' 

 of sections of species of Thuya and some species of Cupressus 

 reveals the presence of pits in the field with an almost vertical 

 pore, and the variation in the breadth of the border and in the posi- 

 tion of the pore is too great to admit of more than a restricted and 

 cautious use of this anatomical feature as a means of distinguishing 

 geneia or even families. It is by no means easy even in sections 

 of recent woods to observe with accuracy the structure of the ray 

 pits : in many cases they are more or less bordered, but the greater 

 distinctness of the pore often leads to the neglect of the fainter 

 border. Moreover the small medullary-ray pits may be con- 

 verted into large pits by the action of fungal hyphae. The large 

 pits of some Pines, Sciadopitys, etc., represent one extreme; 

 intermediate types are represented by Cedrus, Taxodium, and 

 Glyptostrohus, while in Juniperus and several other genera the pits 

 are smaller and more numerous^. 



The depth of the rays as seen in tangential section is a feature 

 to which much attention has been paid, but this is a very variable 

 and comparatively unimportant character^. In a single species 

 of Abies the depth varies from 1 to 63 cells*. Many authors in 

 describing fossil wood state the number of rays per square milli- 

 metre of a tangential section. Characters such as these may 

 undoubtedly be useful in certain cases if used in conjunction with 

 others, due allowance being made for the range of variation within 

 the limits of a single stem. A more important feature is the 

 occurrence of broad rays containing horizontal resin-canals such 

 as those of Pinus, Picea, Larix, and Pseudotsuga. Another useful 

 criterion is afforded by the association of horizontal tracheids 

 (fig. 693, B) with the parenchyma of a ray usually at the upper 

 and lower margin but sometimes, e.g., Pinus canariensis*, in the 

 middle. The occurrence of such tracheids was formerly regarded 

 as a trustworthy distinguishing feature of the Abietineae with the 

 exception of Abies and Pseudolarix, but they are now known to 



1 PenhaUow (07). 2Essner(86); Barber (98). 



3 Fujioka (13). * Strasburger (91) p. 21. 



