MEDULLARY RAYS 



95 



From this it would appear that Cordaites as a whole approaches 

 the primitive, multiseriate ray, such as may be found in the 

 Cycads, much more nearly than any of the existing species under 

 consideration, and from this point of view it becomes possible 

 to arrange a sequence showing the relative development in the 

 following terms: (i) Cordaites, (2) Libocedrus, 

 (3) all other genera, as enumerated above. The 

 evidence of fossil plants, however, shows that 

 caution must be exercised in our estimate / Q 



of what constitutes the primitive ray. The 

 structure of Stigmaria shows a preponderance 

 of one-seriate medullary rays (81, 224), and 

 that such are primitive rays cannot well be 

 doubted. In general, however, we are prob- 

 ably not far from correct in the assumption 

 that the highest form of the ray is expressed 

 in its one-seriate character. Deviations from 

 this would then require to be interpreted as 

 vestigial features, which indicate a relatively 

 lower type of organization in direct propor- 

 tion to the increase of a tendency toward a 



\ I 



multiseriate form. W/ 



In the majority of species the side walls of 

 the parenchyma cells are thick and traversed 

 by small pits. In the genus Pinus the wall 

 is commonly thin, and it closes the orifice of FlG - 2 9- PINUS RE- 



FLEXA. Tangential 



a very large pit on the wall of the adjacent sec tion of a medui- 

 wood tracheid. This is notably true of the 

 soft pines, in which the side wall either pro- 

 jects as a convex membrane, or it is concave 

 and curves into the cell cavity. Such a feature is of very little 

 if any importance except in P. reflexa, where the thin side walls 

 almost invariably project so as to give the cells a correspond- 

 ingly inflated appearance (fig. 29). It is not only apparent in a 

 tangential section, but is very conspicuous in the transverse sec- 

 tion (fig. 1 8), where the inflated walls are seen to project into the 



lary ray showing the 

 typically inflated 

 cells. X 300 



