MEDULLARY RAYS 



79 



are (fig. 18). Such a feature is of specific value in differentia- 

 tion. With this exception the ray presents no features in this 

 plane of section which merit special consideration. 



Radial section. Viewed radially, the medullary ray is seen to 

 be composed of a series of cells extended in a radial direction and 

 superimposed so as to form a muriform band from one to many 

 cells in height. In general terms, the higher the ray the lower 

 the component elements, from which it follows that in one-celled 

 rays the cells are usually highest ; but this 

 feature is only of general interest, since 

 it rarely has a bearing upon the chief ques- 

 tions at issue. In some cases two struc- 

 tural types may be recognized, - - the one 

 containing resin passages, the other devoid 

 of such structures. Where such passages 

 occur the structure of the ray shows a 

 variation of detail which makes it of no 

 value for diagnostic purposes, and the rela- 

 tion is one which possesses interest only 

 in so far as it applies to the distribution of 

 the resin passages themselves. 



A feature of primary importance in the 

 constitution of the ray is the occurrence 

 of two kinds of parenchyma cells. In 95 

 per cent of the genera the upper and lower 

 walls are always thickened by secondary 

 growth and more or less strongly perfo- 

 rated by simple pits (figs. 19 b, 25, and 27). 

 This feature also applies to 56.1 per cent of the genus Pinus. 

 It possesses no special value for either specific or generic differ- 

 entiations except so far as it applies to cells which are markedly 

 different and justify the special terms thick walled23\& thin walled. 

 It is obvious, then, that the thick-walled cell is to be regarded as 

 the normal structure for the ray of the Coniferales as a whole, 

 while the thin-walled represents the exceptional form which is 

 introduced in response to some special demands. Although the 



FIG. 18. PINUS REFLEXA. 

 Transverse section of 

 a medullary ray show- 

 ing the inflation of the 

 cells opposite tracheids. 

 X 300 



