The Origin of Ray Tracheids in the Coniferae

1. In the young root complete transitions may be observed from short tracheids extending between the rays, to ray tracheids both marginal and interspersed. In the young stem only remnants of the transition usually remain. The complete series, however, may occur traumatically. 2. Further evidence of the origin of ray tracheids from tracheary tissue is found in (1) their development in the cambial region of the young plant, (2) the occurrence of tail-like projections, and (3) the possession of tertiary spirals. 3. The occurrence of ray tracheids bears a definite relation to that of albuminous cells. 4. In Abies the possession of albuminous cells and the traumatic occurrence of ray tracheids indicate that the latter are vestigial. 5. The regional and fossil distribution of ray tracheids indicates their ancestral absence in the pines. 6. The hard pines are more specialized than soft ones. 7. The large rays of Pinus are usually formed by the fusion of smaller ones. 8. Ray tracheids are often replaced by parenchyma cells. The importance of this in the formation of secondary parenchymatous rays has been indicated.