No. 448.] NORTH AMERICAN CONIFERALES. 26 1 



Between 



the strictly i -seriate, and those 



strictly 2-seriate type, there is an intermed: 

 group comprising 34 species, or 29.3 % of 

 species, into which members of the first 

 two groups may occasionally be pro- ||| 

 jected. The distinguishing feature of \\\ 

 this group is the occurrence of pits in I I 

 pairs which are usually distant, and in 1 | 

 no case so numerous as to distinguish 

 a 2-seriate disposition. They give un- 

 doubted proof of the passage from one 

 type to the other. Like the 2-seriate 

 type, this feature is not confined to any 

 one genus or to any particular group of 

 genera, but it applies with equal force to 

 any genus, the members of which may 

 therefore represent any or all of the three 

 types here specified. 



Viewing the distribution of the bor- 

 dered pits from the standpoint of zonal 

 development, it is found to be universally 

 true that, in the earlier spring wood there 



is the strongest tendency 

 arrangement. With a rad 

 xylem, this tendency constar 

 the general result that the 

 strictly 



.Is) 



Pi) 



summer wood in which they 

 wholly obliterated — this being 

 the cell wall acquires unusual thi 

 Upon careful examination, the 

 will be found to be in exact acco 

 formulated by De Hary with reference 

 tions in the structure of spu-al tracheid: 

 genesis of bordered pits as already stated. I 

 thow- accordance with this law it is jiossible t(; coi 

 dpUs elude that relatively raj^id growth is coordinate 

 with a primitive development, while the con\ei>- 



,-ith the law 



the 



