i68 
Holden. — Reduction and Reversion in the 
each side of the trace the vessels are surrounded by a jacket of paren- 
chyma. Higher up in the stem biseriate rays disappear entirely except in 
immediate relation to the leaf-trace, while vasicentric parenchyma is scantily 
present throughout. 
Another poplar with the same type of normal wood is P. rotundifolia. 
Here the wound reactions are what might be expected— multiseriate rays 
and abundant vasicentric parenchyma, while a moderate amount of ray 
parenchyma subtends the trace. 
One species remains to be mentioned, the cottonwood of California, 
P. Fremontii. The wood of this species, represented in PI. XXI, Fig. 13, has 
normally biseriate rays, but the parenchyma is exclusively terminal. 
To sum up, the normal wood of the majority of poplars has uniseriate 
rays and terminal parenchyma. Investigation of regions characterized by 
ancestral conditions shows that vasicentric parenchyma is retained by the 
root and seedling and recalled in post-traumatic tissue in the stem. Further, 
multiseriate rays are retained in the seedling, and in connexion with leaf- 
and root-traces, and are likewise recalled by wounding. 
Conditions in Salix are to a large extent similar to those in Popidus. 
Figs. 14, 15, and 16 show three planes of Salix Nuttallii with uniseriate 
rays and terminal parenchyma. Fig. 17 represents a radial view demon- 
strating the vertically elongated ‘ marginal ’ cells of the rays. These cells 
are a perfectly constant diagnostic feature, and serve to differentiate between 
Salix and Popidus. 
Seedlings of various species of Salix show more or less abundant vasi- 
centric parenchyma near the pith, but no biseriate rays except in immediate 
relation to a leaf-trace. Roots show vasicentric parenchyma normally and 
an abundance of biseriate rays after wounding. Both leaf-trace, Fig. 18, 
and root-trace, Fig. 19, show abundant subtending multiseriate rays. 
Wounds in the stem result in the appearance of parenchyma around 
the vessels and multiseriate rays. Fig. 20 shows the wood of 5. nigricans 
var. primulifolia after wounding. Almost every vessel in the field has at 
least one parenchyma cell on its tangential wall and some have two. Fig. 21 
shows a portion of the same under a higher degree of magnification, Fig. 22 
under a still higher magnification. This species gives an especially good 
wound reaction, parenchyma about the vessels being formed to a less extent 
on the side of the stem opposite the wound. 
Young twigs a year old show considerable parenchyma around the 
vessels opposite the protoxylem clusters. At the node a large medium 
trace goes off to the bud, and two lateral ones to the leaf. The bud-trace 
leaves an extensive gap, some distance above and below which are clusters 
of bi- and triseriate rays. The leaf-traces often have subtending them 
rays four or five cells wide, which, half-way through the annual ring, are 
dissected into biseriate rays by the intrusion of vessels. 
