346 



THE AMERICAN NATURALIST. [Vol. XXXVIII. 



glabra show that the interspersed tracheids are not only numer- 

 ous, but that they eventually become conspicuously predominant 

 and often constitute the bulk of the ray structure. It is evident 

 then that such features possess an evident value for diagnostic 

 purposes, particularly in the genus Pinus where the variations 

 are numerous, well defined, and applicable to particular species 

 or groups of species. 



Tangential Section. — As displayed in tangential section, the 

 medullary ray exhibits two principal forms, each of 

 which presents features of great taxonomic and 

 phylogenetic value. The type of structure which 

 prevails, and which may be regarded as the funda- 

 mental form of the ray, is that of from one to 

 many cells superimposed in a single series of vary- 

 ing height (Fig. 25). Such uniseriate rays are 

 characteristic features of all the investigated recent 

 genera. In 30 of the genera, there is a sporadic 

 tendency to a multiseriate form as expressed in the 

 development of rays which are 2-seriate in part. 

 Such enlargement is not confined to any particular 

 portion of the structure, and within the limits of 

 the same section it may arise at the 

 centre or at either end. It is never 

 found in Abies, Picea or Pinus, but it 

 is met with in Pseudotsiiga niacro- 

 carpa, three species of Cupressus, two 

 of Juniperus, one each of Sequoia and 

 Fig. ^s.-seguoia Araucaria and two of Larix (Fig. 26). 

 'iZitlZr^z- Libocedrus such tendency is much 

 tion of a medul- more pronounccd, and the rays may 

 l^^pkanyHeri^ be described as 2-3-seriate in part, 

 form^^x^sr^^ '^^^^ feature is of so sporadic a 

 nature that existing species afford no 

 satisfactory evidence as to its origin or signific 

 but reference to Cordaites tends to throw 

 light upon this somewhat obscure problem. In 

 fourteen species of Cordaites, three of which are 

 European (25, p. 606-609) it is seen that the rays 



some brevifolia- 



