No. 449.] NORTH AMERICAN CONIFERALES. 



355 



primitive structure, a feature which is less perfectly expressed in 

 such transitional forms as P. koraiensis or P. inops. But a mere 

 mingling of the two kinds of pits in the same species is not the 

 only evidence in this direction. The mingling of simple and 

 bordered pits does not occur indiscriminately, but in accordaiu c 

 with a well defined law to the effect that the former are i har- 

 acteristic of the spring wood throughout its entire extent, while 

 the latter occur only in the summer wood where the\ ini,-ht he 

 expected if at all, since the arrested develoiMiient whic h nii-ht 

 be complete in the case of relatively thin-walled c ells, rould he 

 readily overcome in part, in walls of greater seeondary -rcwth. 

 This in no way conflicts with the observed taet that in the 

 majority of cases, the usual course of developnient is sin h that 

 the bordered pits of the spring wood very commonly heeonie 

 reduced to simple pits in the summer wood in accordance with 

 De Bary's law as already stated in application to other cases. 

 Constancy in the structure of such pits has been found to be 

 characteristic of Cordaites, Ginkgo, the Taxaceae and all the 

 lower forms of the Coniferae, from which we may conclude that 

 the bordered pit is essentially a primitive character. On the 

 other hand variation is a well marked feature of the i)it in the 

 genus Pinus as first expressed in the large, oval or squarish and 

 open pits of P. resinosa or P. tJmnbcrgii, and as later api)cars 

 with greater frequency in the smaller and very inconstant pits 

 of P. tceda or P. palustris. Such variations then, involving 

 a gradual and complete transformation tf) the C(jndition of 

 simple pits, are characteristic only of the more highly dex elojied 

 pines, from which it may be concluded that it is a feature con- 

 sistent with a relatively high order of development m exact 

 accord with the principles governing parallel changes ni the pits 

 of the wood tracheids. They are also in harmony with the well- 

 known principle that variation is always of a more simphtied 

 form in primitive types, but that it tends to greater du ersi lie a- 

 tion with advance in organization and general development, as a 

 necessary sequence to the adjustment of the organism to a wider 

 and more complex environment. Finally it has been shown that 

 the elimination of the bordered pit proceeds concurrently with 

 the more complete organization of the ray tracheids, in resi)onse 



