No. 448.] XORTH AMERICAN COAIFKRA LKS. 



'47 



ary growth in thickness of a local nature — the localization 

 being determined with reference to the requirements of sucli 

 support in the first instance. De Bary (9, p. 57) has shown 

 that they exhibit considerable variety in the number ol tihrcs 

 and the direction and steepness of the coils. Their nunibci is 

 often only 1-2 in narrow tubes which are first formed when the 

 differentiation of tissues begins, in others 4 or more, and il rises 

 in many cases in the angiosperms to 16-20. He has furthcrniorc 

 pointed out that the steepness of the coils is greatest in those 

 tubes which are developed earliest, before the extension of ihc 

 part to which they belong has ceased; since in these the coils 

 are separated from one another by the extension which the tube 

 itself undergoes. These facts appear to suggest that the more 

 typical the spiral tracheid is, the more fully does it emjihasize 

 the idea of a primitive structure ; but that as the spirals become 

 more dense or closer, there is a tendency toward more uniform 

 and less localized secondary growth of the wall, as expressed ni 

 the structure of the higher types of plants or the secondary 

 xylem elements of the Coniferae. In confirmation of such a 

 view, reference may be made to the commonly observed fact 

 that the spirals tend to a more compact arrangement at the ends 

 of the elements, becoming correspondingly more open in the 

 central region; and likewise to the well known transitional 

 forms which these structures exhJbit, whereby their original 

 characteristics are completely lost as they merge ultimately into 

 tracheids devoid of spirals, but characterized by the presence of 

 pits of various forms. In 1840, Don (52) pointed out that the 

 tracheids of Cycas revoluta exhibit scalariform structure at one 

 end and bordered pits at the other. This fact has more recently 

 been commented upon by Williamson who observed the same 

 fact independently, and drew from it the inference that a defi- 

 nite relation exists between the scalariform markings and the 

 pit structures of such a nature that the one is the natural 

 successor of the other. In Ginkgo biloba which is now gener- 

 ally conceded to represent a much more primitive type than the 

 Coniferee, though more advanced than the cycads, precisely 

 similar transformations are to be met with. The evidence of 

 fossil plants is quite as convincing as that derived from. existing 



