1910] THOMPSON—RAY TRACHEIDS II5 
is strengthened by the earlier and more rapid development of ray 
tracheids in Pinus resinosa than in Pinus Strobus. 
Summary 
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 (r) 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 par enchyind cells. The 
importance of this in the formation of secondary parenchymatous 
rays has been indicated. 
This study was undertaken at the suggestion of Mr. R. B. THom- 
SON and carried on with his constant advice. My warmest thanks 
are due to him for quite exceptional kindness throughout the course 
of the work. 
UNIVERSITY oF ToRONTO 
LITERATURE CITED 
1. PENHALLow, D. P., North American Gymnosperms, 1907. a 
- Jerrrey, E. C., ‘isceiate ray tracheids in Cunninghamia sinensis. 
of Botany 22: a hae pl. 31. 1908. 
3. Kwy, L., Anatomie des Holzes von Pinus silvestris. Berlin. 1884. 
