1913] CHRYSLER—PHLOEM OF ABIETINEAE 47 
sisting in the former which have been lost in the latter, that is to 
say, marginal cells occur in the xylem and phloem of J. communis, 
while they are restricted to the phloem of J. virginiana. If, as 
recent studies indicate, the Cupressineae are derived from large- 
leaved conifers such as the Abietineae, the persistence of marginal 
cells in the phloem after they have disappeared from the xylem 
must be interpreted as the retention of an ancestral feature, which 
amounts to saying that the phloem is a comparatively conserva- 
tive region. 
The genus Cedrus shows in a marked way the belated appearance 
of the ray tracheids compared with the erect cells, but a detailed 
consideration of Cedrus and Pseudolarix is reserved for separate 
treatment in a future paper. 
The choice of Pinus for detailed study has been made on the 
basis of its probably primitive nature, in the light of recent paleon- 
tological discoveries. If the origin of the erect cells is established 
for Pinus, it holds for the other genera of Abietineae, though they 
may not so clearly show the formative stages. Within the genus 
Pinus it appears from the work of Barry (1) that the nut and 
foxtail pines are to be regarded as the most primitive, hence 
P. edulis, P. cembroides, and P. aristata from Arizona were studied 
with especial interest. Although difficulties were experienced in 
judging the ages of roots, and seedlings were not available, the 
material showed the same appearances as had been observed in the 
eastern pines, as several of the figures indicate. One point was 
established which forms an additional] argument for the primitive 
nature of these species, namely, the marginal cells in the xylem 
make their appearance considerably later than in the eastern pines 
investigated. Since marginal cells are absent from the reproductive 
axis and from the early rings of growth in stem and root, and 
particularly because they are not found in the older Pityoxyla (3), 
they are regarded as of comparatively recent introduction, and it 
- follows that those species in which they appear late in development 
are to be considered primitive, unless other evidence indicates the 
contrary. In respect to the time of appearance of the marginal 
cells, the nut pines and certain of the hard pines, such as P. rigida, 
stand at opposite extremes, according to the limited study which 
this point has received. 
