[pKNHALLowJ ^'OTES ON TERTIARY PLANTS 65 



aged, large, tubular trachea? whereby the internal cavity is partially 

 or completely filled with parenchymatous cells which, when their 

 course of development is once established, may continue their growth 

 after the manner of a tissue. They are familiar structures in the 

 Monocotyledons and in the Dicotyledons, especially in those having 

 long-lived, woody stems; and I have also found them in the vessels of 

 Pteris aquilina where they offer a striking exception to the general 

 law of distribution. Their oecurrence in plants is, on the whole, 

 of such a nature that their development is associated with a 

 higher type of development in the organism as a whole, and 

 the principle thus indicated may be applied with propriety to 

 smaller groups. In the Coniferae the thyloses make their appearance 

 in the resin passages of certain genera where they follow the same 

 course of development as already noted for vessels, and inasmuch as 

 their appearance is related in the first instance to the age of the 

 structure, they may likewise be interpreted as factors indicative of 

 relative degrees of development whenever they become constant. 



In Picea thyloses are developed definitely in P. nigra, P. alba, 

 P. pungens and P. sitchensis. In Pseudotsuga they are developed 

 but sparingly, while in Larix they are apparently wanting. These 

 variations appear to be related to the thickness of the walls in the 

 epithelium cells in such a way that the thinner the walls the more 

 generally are thyloses developed. The pines normally show thyloses 

 in the resin passages, a feature which is so constant and so generally 

 absent from the other genera as to constitute a differential feature of 

 some value. 



Sequoia burgessii from the Lignite Tertiary presents a remarkable 

 example of the development of thyloses in the resin canals of medul- 

 lary rays (Fig. 7), and their occurrence in this species becomes of 

 special interest from a biological point of view. 



Ray Cells of Two Kinds. — The only other structural feature which 

 calls for comment in this connection, is the occurrence of two kinds 

 of parenchyma cells in the medullary ray. This relates entirely to 

 the genus Pinus where it is again confined chiefiy to the section of 

 hard pines. The distinctive difference relates to relative thicknesses 

 of the cell wall whereby one is thin and not pitted, while the other 

 is thick and pitted as in the soft pines. 



If upon the basis of the foregoing considerations, the various 

 genera are arranged according to the law of frequency, they will fall 

 in the order given in the subjoined table, where the sequence expresses 

 the approximate relations in development as based upon purely struc- 

 tural grounds, but without attempting to finally determine the precise 



Sec. IV., 1903. 5. 



