CONIFERALES 



243 



of a condition which later became prominent; while Jeffrey,-""* 

 basing his conclusion upon their appearance in wound tissue of 

 Cunninghamia sinensis, thinks the rare cases are atavistic. 



Ray tracheids have the position of ordinary cells of the ray, to- 

 gether with much of the structure of tracheids. Some think they 

 are moditied cells of the ray, while others claim that they are modi- 

 fied tracheids. Thompson^^s Y^^iS presented the principal evidence in 



Fig. 249. — Pimis resinosa: a series of tracheids in a wounded region, showing trans- 

 formation to ray tracheids. — After Thompson.^-s 



favor of the latter view. In the adult stem of Pinus resinosa, a series 

 of tracheids from a wounded region showed various gradations be- 

 tween ordinary tracheids and the usual ray tracheids; and in the 

 cone axis of Pinus strobus, which is not supposed to have any ray 

 tracheids, the end of an ordinary tracheid is sometimes bent along a 

 ray (figs. 249, 250). 



Besides these narrow rays, usually only one cell wide, there are 

 rays several cells in width, broad enough to contain a resin canal. 

 Similar rays, in the cycads, contain a mucilage canal, and, below it, 

 a leaf trace, variously connected with the lower part of the rays. In 



