MEDULLARY RAYS 



101 



Generic. 



diagnosis can be drawn only when we take cognizance of the 

 principal aspects presented, and these are sometimes as many 

 as four in number. 



A consideration of the various structural features thus dis- 

 cussed in their relations to classification will show that no other 

 portion of the stem possesses so many elements of importance as 

 the medullary ray, which, in consequence, attains the highest 

 value in this respect and affords differential characters of wide 

 range, great prominence, and easy recognition, and is of primary 

 importance in the differentiation of groups, genera, and species ; 

 and, as a general summary, the utility of these characters for such 

 purposes is approximately indicated in the following tabulation : 



1. Rays (tangential) of two kinds. 



2. Ray tracheids. 



3. Pits on the lateral walls of the ray cells simple or bordered. 



4. Terminal walls of the ray cells thin and entire or locally 



thickened. 



5. Form and character of the ray cell (tangential). 



6. Form and size of pits on the lateral walls of the ray cells. 



7. Ray tracheids dentate or reticulated. 



8. Direction and form of orifice of pits on the lateral walls of 



ray cells. 



9. Upper and lower walls of ray cells. 



10. Ray tracheids interspersed or marginal. 



11. Disposition of pits (radial). 



12. The number of pits per tracheid. 



The marginal cells of the ray- - that is, those which terminate 

 the ray above and below, as seen either in a radial or a tangential 

 section --are usually somewhat different from those which con- 

 stitute the bulk of the structure. This difference is expressed in 

 a tangential section by their greater height and relatively narrower 

 form. In a radial section it is expressed by the greater height? 

 the somewhat thinner walls, and the sinuous form of the latter as 

 they conform to the terminations of adjacent tracheids. In such 

 deviations from the usual characteristics of the ray cells it is 

 probable that those of the margin may be held to exhibit a tend- 

 ency to the formation of ray tracheids of which they would 



- Specific. 



