102 ANATOMY OF THE GYMNOSPERMS 



therefore be regarded as potential forms. This view gains 

 strength from the fact that the ray tracheids first appear in just 

 those situations ; that when they are sporadic, as in Abies bal- 

 samea, they are interspersed and conterminous with the marginal 

 cells ; that their external forms and general aspect are the same ; 

 and that when the tracheids are fully developed and become 

 constant features of the ray it is at the expense of the ordinary 

 marginal cells, which then disappear. Furthermore Jeffrey has 

 shown that in Sequoia Penhallowii (25), where the marginal cells 

 assume a very characteristic form, they are also interspersed in 

 the higher rays precisely as tracheids are in the rays of the higher 

 Coniferae. Another feature of these cells --to which Jeffrey 

 has directed attention in Sequoia Penhallowii - - is the presence of 

 numerous crystals. This is unique among the Sequoias, and it 

 is unknown in any other genus of the Coniferales except Abies, 

 where, as Jeffrey also shows, a similar deposit of crystals is to 

 be met with in A. concolor, A. grandis, A. bracteata, A. nobilis, 

 and A. magnifica ; but it is a feature of much more sporadic 

 occurrence, since large areas of these species show no crystals, 

 while in Sequoia Penhallowii they are exceedingly abundant. 

 Jeffrey correctly regards this as indicating a certain relationship 

 between these two genera (25), a connection, however, which is 

 also indicated by other structural features, as pointed out by 

 Penhallow some years since (44, 45). 



