1913] CHRYSLER—PHLOEM OF ABIETINEAE 39 
amitosis), and finally become completely collapsed during their 
second year, after the manner of sieve tubes. On the xylem side a 
few short tracheids are cut off, but their walls are thin, and they can 
be traced for only about the width of two tracheids. 
Cells in radial groups are frequent in the phloem of young roots 
in all the species of Pinus which have been studied, and their 
occurrence has been recorded by STRASBURGER (5), who states that 
they are characteristic of the young Rane 
regions of the plant, before the tan- 
gential rows of phloem parenchyma 
cells become established. The pres- 
ent investigation goes to show that 
the radial groups of cells are not only 
more abundant in the young regions, 
but that they are more common in 
young roots thanin stems. It should 
be stated, however, that the cells of 
the radial rows do not for the most 
part correspond to phloem paren- 
chyma, as will presently appear. In 
many cases the cambium for a period 
of one or more years cuts off no cells 
on the xylem side, but generally from 
the median region of such radial 
gtoups a narrow medullary ray is de- 
veloped, as is clearly shown in fig. 3; 
which is drawn from a 5-year root of Fic. 2.—P. cembroides, 8-year 
P. resinosa. Cambial activity dwin- root: radial group of cells provided 
dles until it is confined to the median With — . re 
region of the radial group, and a med- Sail alse Wess: es 5. : 
ullary ray one cell in height is formed. 
It will be noticed that in the xylem region this ray consists of 
tracheids, though cases occur where the ray cells first formed 
‘are tracheids and the later ones are parenchymatous, as shown by 
the pitting. Thus a tracheidal ray may turn into a parenchyma- 
tous one, but the reverse seems to happen seldom if ever. Atten- 
tion has been called to this point by THompson (7). The initial: 
we 



