No. 449.] NORTH AMERICAN CONIFERALES. 



333 



recognized — the one containing resin passages, the other devoid 

 of such structures. Where such passages occur the structure 

 of the ray shows a variation of detail which makes it of no 

 value for diagnostic purposes, and it is therefore eliminated from 

 the following discussion. 



A feature of primary importance in the constitution of the 

 ray is the occurrence of two kinds of parenchyma cells. In 

 95% of the genera the upper and lower walls are always thick- 

 ened by secondary growth and more or less strongly perforated 

 by simple pits (Figs. 17^, 21 and 23). This feature also applies 

 to 56.1 '](. of the genus Pinus. It possesses no special value for 

 either specific or generic differentiations except so far as it applies 

 to cells which are markedly different and justify the special 

 terms " thick-walled " and "thin-walled." It is obvious, then, 

 that the thick-walled cell is to be regarded as the normal struc- 

 ture for the ray of the Coniferales as a whole, while the thin- 

 walled represents the exceptional form which is introduced in 

 response to some special demands. Although the thick- walled 

 cells occur in the genus Pinus to the extent of 56 they show 

 a diminishing frequency, eventually becoming rare and ultimately 

 replaced by thin-walled cells. Reference to them in the follow- 

 ing diagnoses is always specified by (i). In 43.9 ^- of the genus 

 the upper and lower walls are thin and absolutely devoid of pits. 

 For diagnostic purposes such cells are always referred to as (2). 

 In some cases they are so undeveloped as to be obscure and 

 readily broken out in the process of section cutting, so that they 

 are often entirely wanting (Fig. 17^). Such thin-walled cells 

 are typically developed in P. palustris, P. tmia, etc., and 

 it is to be observed that they are always associated with the 

 highest forms of development. Transition forms occur. These 

 are first seen in the soft pines where occasional thin-walled cells, 

 devoid of pits, are interspersed and often coterminous with the 

 thick-walled elements. In the hard pines the same relation 

 exists, but it is gradually reversed until the thin-walled cells 

 altogether predominate. Such gradations are exhibited in P. 

 coiilten, P. Jeffrey i, P. pungens, P. tceda, P. cub ens is and P. 

 inops, and they afford evidence of value as to the sequence in 

 development of the species. In murmyana, P. cubensis and 



