[PENHALLow] N. AMERICAN TAXACE^ AND CONIFERS 39 



then show a coarsely pitted structure similar to that of a poorly formed 

 sieve-plate. The general law of distribution shows that in those woods 

 which have well defined resin-passages the resin-cells are wanting. Simi- 

 larly, those woods which have an abundance of resin-cells show an ab- 

 sence of resin-passages — the one replaces the other. Exceptions to this 

 law naturally occur. Thus, in Pseudotsuga and Larix, genera which are 

 distinguished by their prominent resin-passages, there are also well de- 

 fined resin-cells. So also in Sequoia and Abies, genera conspicuous for 

 their resin-cells, resin-passages sometimes occur. 



Our investigations show that in all genera having resin -passages in 

 the wood there are also resin-passages traversing the stem in a radial 

 direction and embraced in certain of the medullary rays, which have 

 their general form and structure correspondingly altered. Under such 

 circumstances the ra3's become, as a rule, much higher and alwaj^s much 

 broader than the ordinary rays. The modification, as exhibited in a tan- 

 gential section, is such that, while the terminals above and below are 

 acute or linear, the central tract is broadened out more or less abruptly, 

 and then consists of one large resin-passage, and usually also of much 

 reduced parenchymalous-cells, lying immediately external to the epithe- 

 lial structure, thus forming the outer limits of the tract. Such rays, 

 which, from their form, may be designated as fusiform, in order to read- 

 ily distinguish them from those of the ordinary linear and uniseriate 

 type, are always found in association with resin-passages, which traverse 

 the stem longitudinally. So intimate is this relation that the presence of 

 one may alwa3^s be inferred from the other. All North Ameiican species 

 of Taxacece, without exception, show a complete absence of all three of 

 the elements so far considered — resin-cells, resin-passages and fusiform 

 rays. It thus becomes possible, on these grounds alone, to definitely 

 separate this familj- from all the Conifene. Among the latter the genus 

 Pseudotsuga stands out prominently as an almost unique instance of a 

 ease approaching the Taxaceœ in one of its most salient features. In all 

 of the North American Taxaceœ, without exception, the tracheids are 

 characterized by the presence of a double series of spiral bands. So dis- 

 tinctive are these structural features that, with one exception, they in- 

 variably point to a member of this family. In the genus Pseudotsuga 

 similar spirals are to be met with as a constant element of structure, with 

 this difference, however, that while in the Taxaceœ the spii'als are a con- 

 stant element of all the tracheids, in Pseudotsuga they are often entirely 

 absent from the summer wood ; they are, nevertheless, always to be met 

 with in the spring wood. Any confusion which might otherwise arise 

 through the presence of such spirals is at once i-emoved by the fiict that 

 whereas in Taxacea- there are no resin-passages or fusiform rays, both of 

 these structui-es are characteristic of Pseudotsuga. Occasionally other 

 conifers manifest a tendency to the formation of spirals. Thus in Larix 



