CHAPTER V 



MEDULLARY RAYS 

 GENERAL STRUCTURE 



The medullary ray, in the various details of its structure, 

 as presented radially and tangentially, comprises some of the 

 most important features for diagnostic and taxonomic pur- 

 poses. While it presents numerous variations, these are, in the 

 main, of such a nature as to give them very positive value for 

 both generic and specific differentiations. Primarily the medul- 

 lary rays are to be regarded as a residue of the original funda- 

 mental structure, which has been left over in the genesis of the 

 primary stele, but they are capable of reproduction or extension 

 under the influence of the cambium in the course of secondary 

 growth. In all such cases, however, they are typically composed 

 of the same elements which are necessarily parcnchymatous. 

 Deviations from this structure may appear through the intro- 

 duction of other elements, but such alterations always arise in a 

 manner which indicates their relation to the evolution of higher 

 types of organization. 



In a transverse section the medullary ray appears usually as 

 a simple, radial series of elongated cells with transverse termi- 

 nations. Deviations from this type of structure occur only in 

 the case of the rather rare two-seriate forms, which appear in 

 the transverse plane of section only at wide intervals, or in the 

 case of rays which contain resin passages, as in Picea, Pinus, 

 Larix, etc., when the structure presents a varying aspect depend- 

 ent upon the particular plane of section. In Pinus reflexa the 

 side walls of the ray cells may be seen projecting into the 

 cavities of adjacent tracheids, where they form short, saclike 

 bodies of the general nature of thyloses, which they really 



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