MEDULLARY RAYS 



8 



(fig. 23). It occurs exceptionally in Juniperus and Finns, but it 

 is typical in Abies (90.9 per cent), Tsuga (100 per cent), Larix 

 (100 per cent), and Picea (90 per cent). In Abies and Juniperus, 

 where transitional forms sometimes occur, the walls in the spring 



FIG. 22. CUPRESSUS MACNABIANA. Medullary ray showing the form and position 

 of the pits ; the thin, curved, and locally thickened terminal walls, x 280 



wood may be only locally thickened, but in such cases the typ- 

 ical feature always appears in the summer wood, where such 

 secondary alterations are most strongly emphasized. 



For taxonomic purposes such features possess a definite value. 

 The thick-walled cells of Tsuga, Larix, and Picea permit of an 



FIG. 23. JUNIPERUS OCCIDENTALIS. Medullary ray showing the form and dispo- 

 sition of the pits on the lateral walls ; the thick and coarsely pitted terminal 

 walls, x 280 



easy and definite segregation of these three genera in those cases 

 which otherwise might involve a strong element of doubt, and 

 the same rule holds true, though to a less extent, with respect 

 to the locally thickened walls in Cupressus and related genera. 

 Pits on the lateral walls of the ray cells are an invariable 

 feature of all investigated species of Gingkoales and Coniferales, 



