6 4 



ANATOMY OF THE GYMNOSPERMS 





becomes more or less strongly elliptical, but as the segregation 

 is more pronounced a definitely circular form prevails (fig. 10). 

 Within the limits of the individual tracheid the same law of dis- 

 tribution obtains as in the two-seriate type, whereby segregation 

 is always most pronounced in the central region. 



Between species of the strictly one-seriate and those of the 

 strictly two-seriate type there is an intermediate or transition 

 group comprising thirty-four species, or 29.3 per cent of the in- 

 vestigated species, into which members of the other two groups 



may occasionally be projected. The distin- 

 guishing feature of this group is the occur- 

 rence of pits in pairs, which are usually 

 distant and in no case so numerous as to 

 distinguish a two-seriate disposition. They 

 give undoubted proof of the passage from one 

 type to the other. Like the two-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 bordered 

 pits from the standpoint of zonal devel- 

 opment, it is found to be universally true 

 that in the earlier spring wood there is the 

 strongest tendency to a multiseriate arrange- 

 ment. With a radial increase of the xylem this tendency con- 

 stantly diminishes, with the general result that the pits become 

 more strictly one-seriate and more distant toward the summer 

 wood in which they are sometimes wholly obliterated, this being 

 the case when the cell wall acquires unusual thickness. 



Upon careful examination the foregoing facts will be found 

 to be in exact accord with the law formulated by De Bary with 

 reference to variations in the structure of spiral tracheids and 

 the genesis of bordered pits, as already stated. In accordance 

 with this law it is possible to conclude that relatively rapid 



FIG. 10. PINUS STRO- 

 BUS. Radial section 

 showing the bordered 

 pits of the summer 

 wood. X 280 



