MEDULLARY R.\\S 



81 



thin-walled cells altogether predominate. Such gradations are 

 exhibited in P. Coulteri, P. Jeffreyi, P. pungens, P. taeda, P. cu- 

 bensis, and P. inops, and they afford valuable evidence as to 

 the sequence in development of the species. In P. Murrayana, 

 P. cubensis, and P. insignis the transition forms exhibit much 

 more detailed gradations, by virtue of which it is often ex- 

 ceedingly difficult to distinguish between the two forms of cell, 

 since whether conterminous or parallel the variations in thickness 



FIG. 20. TAXODIUM DISTICHUM. Medullary ray showing the structure and posi- 

 tion of the pits on the lateral walls; the straight ray cells and the thin, 

 straight, terminal walls, x 280 



change in such a way that the one type passes gradually into 

 the other. When these variations are viewed collectively and 

 taken together with the general fact that the thin-walled cells 

 are a feature of the higher types of organization, we may reason- 

 ably conclude that the thin-walled cells have been derived from 

 the thick-walled through a process of arrested development. The 

 cause of such alterations is to be sought for, and it will doubt- 

 less be found in connection with another component of the ray. 

 The terminal walls of the ray cells present three typical vari- 

 ations, (i) thin-walled and entire, (2) thin-walled and locally 

 thickened, and (3) thick-walled and coarsely pitted. 1 The first 



1 See Appendix A. 



