THE AMERICAN NATURALIST. [Vol. XXXVIII. 



wall may return to a primitive condition of structure through 

 various phases of atrophy. In accordance with this idea the 

 tracheid would be introduced as the most natural because the 

 original medium for such activities as are centered in the ray, 

 and it would therefore acquire additional importance both nu- 

 merically and functionally in direct proportion to the loss of 

 power experienced by the parenchyma cells. This appears to 

 be a reasonable interpretation, and in the light of observed facts 

 it would seem to be the correct one. 



A structural feature of great importance in the ray tracheid 

 appears in certain inequalities of the upper and lower walls 

 which take the form of teeth-like projections into the cavity 

 (Fig. 24). In what may be regarded as the most highly devel- 

 oped tracheids the teeth project across the cell cavity until they 

 meet and coalesce, thereby forming a more or less definite retic- 

 ulation which gives to the tracheid a very characteristic appear- 

 ance. As seen in tangential section, such reticulations often 

 appear as narrow bands crossing the cavity from side to side, 

 thus giving the cell a varying aspect. Such dentate and reticu- 

 lated tracheids are absolutely confined to the second section of 

 ' the genus Pinus, in which they constitute one of the most char- 

 acteristic features to the extent of 68.3 f> of the species. A 

 more detailed analysis of this feature, as applied to the hard 

 pines, is desirable. In P. resinosa and P. thuubergii, the tra- 

 cheids are simply dentate. In six species represented by P. 

 murrayana the teeth extend into definite reticulations confined 

 to the summer wood ; but in six other species represented by 

 P.jeffreyi, such reticulations are sparingly developed through- 

 out the ray. In P. tceda a transitional form appears. Typically 

 this species shows the tracheids to be sparingly reticulated, but 

 occasionally they are strongly reticulated throughout. This 

 brings to mind the further fact that in all species which are 

 sparingly reticulated there is a marked tendency to strong retic- 

 ulation in the summer wood. In the thirteen remaining species 

 the tracheids are uniformly strongly reticulated throughout the 

 extent of the ray, and this feature attains its highest expression 

 in P. pahistris and P. aibensis. It is therefore manifest that 

 we have to deal here with a graduated development of such a 



