172 
Holden . — Reduction and Reversion in the 
a condition of reduction from a high type. The evidence for this view 
may be briefly stated as follows : The primitive distribution of wood 
parenchyma in the Salicaceae as shown by this investigation is obviously 
vasicentric or centred around the vessels, a condition uniformly charac- 
teristic of such high families as the Compositae, Oleaceae, &c. In ray 
structure, the Salicaceae are likewise clearly allied with the higher and not 
the lower Dicotyledons, since their uniseriate rays obviously represent 
a condition of reduction from the multiseriate type characteristic of the 
higher woody Angiosperms and not a primitively simple condition as 
suggested by the late Professor Penhallow. 
Conclusions. 
1. Most eastern representatives of the North American Salicales are 
characterized normally by terminal parenchyma and uniseriate rays. 
2. In primitive regions vasicentric parenchyma and multiseriate rays 
are found. 
3. Certain western North American representatives of the Salicales 
have normally vasicentric parenchyma and multiseriate rays. 
4. The primitive condition for the wood of the Salicales is vasicentric 
parenchyma and multiseriate rays. 
5. The low position assigned to the Salicales by systematists appears 
not to be justified. 
6. Since their simple structure is due to reduction from a condition 
originally more complex, the Salicales have a high position in the dicotyle- 
donous series. 
In conclusion, I wish to express my thanks to Professor E. C. Jeffrey 
for his helpful advice and for aid in securing the photomicrographs accom- 
panying this article; to Professor J. G. Jack of the Arnold Arboretum for 
seedling material he was kind enough to send me ; and to Mr. I. W. Bailey 
for sections of various western species. 
EXPLANATION OF PLATES XX AND XXI. 
Illustrating Miss Holden’s paper on Salicales. 
PLATE XX. 
Fig. 1. Populus trichocarpa : transverse section of normal wood, showing uniseriate rays and 
terminal parenchyma, x 300. 
Fig. 2. Same : radial section, showing terminal parenchyma, x 300. 
Fig. 3. Same : tangential section, showing uniseriate rays, x 300. 
Fig. 4. P. tremuloides : transverse section of wounded wood, showing biseriate rays, x 300. 
Fig. 5. Same : tangential section, showing biseriate rays, x 300. 
Fig. 6. Same : transverse section of root, showing vasicentric parenchyma, x 400. 
