1910] THOM PSON—RAY TRACHEIDS © 109 
sented. There is a hurrying over the early stages, so that an actual 
series can rarely be observed. Nevertheless, the transitional form of 
tracheid is occasionally found, and has been drawn in fig. 11 from the 
second year’s growth of Pinus Strobus. Tail-like projections are 
quite common, and have been illustrated in fig. 12. 
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Fics, 11, 12.—Fig. 11, P. Sirobus: young stem; transitional tracheids; fig. 12 
P. Strobus: adult stem; ray tracheids with “tails.” 
The only good stem series showing the transformation from 
tracheids to ray tracheids was observed in old stem wood of Pinus 
resinosa which had been wounded (fig. 13). The rapidly shortening 
Series of tracheids ends at the right in true ray tracheids with buttressed 
walls. In this wounded material also the tail-like projections were 
very numerous and very large. Both features are to be accounted 
for as traumatic reversions similar to those described by JErrrey for 
Cunninghamia (2), in which form though ray tracheids are not 
normally present they were found in connection with a wound. 
Even in the normal adult ray tracheid, however, we find indica- 
Hons of its tracheary origin. One of these is the occurrence of tertiary 
