26o THE AMERICAN NATURALIST. [Vol. XXXV 1 11. 



position of the pits as will more fully appear in the following 



Among the remaining Coniferales, 20 species of various gen- 

 era, or 17.2 f/o in all, show a 2-seriate arrangement, and to this 

 group we must also add the Ginkgoales and various fossil spe- 

 cies. Here the multiseriate disposition of the pits involves 

 features which at once distinguish the group as a whole from 

 the preceding, clearly placing, it upon a higher plane of develop- 

 ment. The pits are never hexagonal but generally eUiptical or 

 round, while they also show a strong tendency to extreme seg- 

 regation. When brought into a compact arrangement as in 

 Cupressoxylon, Sequoia or various species of Pinus, they are 

 flattened only along the lines of hmited contact, which are 

 usually confined to one end (Fig. 7). A very characteristic fea- 

 ture of this group is the further fact that the 2-seriate arrange- 

 ment is not constant, either in the same section or in the same 

 tracheid. Both Pinus tisda and P. cubensis, as also Larix ameri- 

 cana and Tsuga canadensis afford illustrations that while typi- 

 cally 2-seriate, a given section may show a strictly i -seriate 

 arrangement, and this difference also obtains as between con- 

 tiguous cells. In all such cases examination will show that the 

 variation is directly related to the relative size of the tracheids 

 in such a way that the narrower tracheids, or those arising from 

 a less vigorous growth, are i -seriate. Within the individual 

 tracheid there is a strong tendency to a i -seriate arrangement 

 in the central region, while it is 2-seriate at the extremities; 

 and this law holds so true that in those species which are excep- 

 tionally 2-seriate, judgment should be reserved until it is seen 

 that the i -seriate form holds throughout. 



The antithesis of the multiseriate type is found in the i-ser- 

 ite form. This is typical of 50 % of all the species included in 

 the present studies. In such cases the form of the pit is never 

 hexagonal or specially flattened. When the disposition is some- 

 what compact, as in Pinus strobus (Fig. 8), the outline becomes 

 more or less strongly elliptical, but as the segregation is more 

 pronounced, a definitely circular form prevails (Fig. 9). Within 

 the limits of the individual tracheid the same law of distribu- 

 tion obtains as in the 2-seriate type, whereby segregation is 

 always most pronounced in the central region. 



