No. 449-] NORTH AMERICAN CONIFERALES. 



portion of the stem possesses so many elements of importance 

 as the medullary ray which, in consequence,[attains the highest 

 value in this respect and affords differential characters of wide 

 range, great prominence, easy recognition and of primary im- 

 portance in the differentiation of groups, genera and species ; 

 and as a general summary, the utility of these characters for 

 such purposes is approximately indicated in the following tabu- 

 lation : 



1 Rays (tangential) of two kinds. 



2 Ray tracheids. 



3 Pits on the lateral walls of the ray cells s 



pie or bordered. 



4 .Terminal walls of the ray cells thin and] en 



or locally thickened. 



5 Form and character of the ray cell (tang.) 



6 Form and size of pits on the lateral walk 



ray 



cells 



7 Ray tracheids dentate ( 



:ulated. 



Direction and form of orifice of pits on t 



eral walls of ray cells. 

 Upper and lower walls of ray cells. 

 Ray tracheids interspersed or marginal. 

 Disposition of pits (radial). 

 The number of pits per tracheid. 



The Relations of Medullary Rays to Development. 



We are now in a position to determine the relations in which 

 the various structural features of the medullary ray stand to 

 development, and for this purpose it may be most convenient to 

 discuss them in that sequence which is apparently cons(;nant 

 with the general order of evolution of the entire grouj). 



It has been ascertained that bordered pits are characteristic 

 features of the lateral walls of the ray cell in 72.4 '/ of the inves- 

 tigated species, and that in the remaining 27.6 '/ among the 

 higher types, simple pits predominate, but a closer scrutiny of 

 this latter group discloses some features of more than passing 



