1909] BAILEY—STRUCTURE OF WOOD IN PINEAE 51 
the year’s growth in plants which have lost, or are in the process of 
losing, their resin canals. In the Pineae, in which resin canals occur 
normally, the parenchyma is confined to the end of the year’s growth, 
and less well developed in the older genera, which have more numer- 
ously developed resin canals. Thus Pinus, with its large supply of 
resin canals, shows only the first steps in the development of wood- 
parenchyma. Septate tracheids occur infrequently upon the outer 
surface of the summer wood, and only in one instance have I been 
able to observe what appeared to be parenchyma in the same location. 
In Picea, with less well-developed resin ducts, the wood-parenchyma 
becomes constant, while in Larix and Pseudotsuga it is usually strongly 
developed. In the Taxodineae and Cupressineae, with the nearly 
complete disappearance of resin canals, the wood-parenchyma is well 
developed throughout the year’s growth, as well as at the end of the 
summer wood. The Abieteae are transitional between the two groups. 
It seems probable, as JEFFREY has suggested in his paper on the Abie- 
tineae (op. cit. 26), that with the reduction of foliage in the Abie- 
tineae and Cupressineae, the freely anastomosing system of canals, 
with its large supply of resinous secretions for sterilizing wounds, 
became too great a burden, and that the system of resin cells was 
gradually developed to take its place. It is not my object to enter 
here into all the various phases of the controversy as to the age of 
the Abietineae, but recent paleontological evidence proves the great 
geological age of Pinus. Further, Prepinus is a primitive abietineous 
type with centripetal wood, resembling certain of the Cordaites, and 
at the same time closely allied to the true pines of the middle Creta- 
ceous.° A survey of all the paleontological evidence, as well as a study 
of the anatomy and morphology of the modern Coniferales, seems to 
show conclusively the greater age of the Abietineae. 
SPIRAL THICKENINGS 
Let us now turn to a consideration of the occurrence and distribu- 
tion of tertiary thickenings of the tracheid walls in the Pineae.’ As 
stated above, PENHALLOW considers spiral thickenings absent from 
© JEFFREY, Structure of the leaf in Cretaceous pines. Annals of Botany 21:207—- 
220. pls. 13, 14. 1908. 
7 It should be kept clearly in mind that these spirals are tertiary thickenings, and 
bene Pa: distinct from the so-called “spiral striations” of the secondary walls of the 
tracheids, 
