North American Salic ales. 167 
The last section includes the end-wall of the cell, and its simple pits prove 
conclusively that it is parenchymatous. 
The other regions retentive of ancestral characteristics confirm the 
evidence supplied by the study of wounds. Fig. 9 represents a tangential 
view of the tissue subtending an outgoing leaf-trace ; Fig. 10 an outgoing 
root-trace. Both show aggregations of bi- and triseriate rays. 
Another species, P. balsamifera , may now be considered. The normal 
wood is similar to that of the two species already described, with uniseriate 
rays and terminal parenchyma. Figs, ix and 12 are of wounded tissue and 
show the presence of biseriate rays. As in P. tremuloides , the wound also 
recalls the parenchyma around the vessels. In the former, however, wounds 
tend to bring back the parenchyma to a much greater extent than in the 
latter ; as regards reversion in the rays, the condition is exactly opposite, 
i. e. a wound which in P. tremuloides would bring multiseriate rays only in 
the immediate vicinity of the injury, in P. balsamifera recalls the rays 
half-way around the stem. In the root of P. balsamifera the parenchyma 
is mostly terminal, with a slight tendency to the vasicentric condition ; 
a very slight wound, however, suffices to bring it back in abundance around 
the vessels. Conditions similar to those of P. tremuloides were found in 
the first annual ring of the stem and under the leaf-trace. 
P. nigra var. italica is another species with terminal parenchyma and 
uniseriate rays. Wounds in stool shoots bring back the parenchyma and 
rays in a striking way, an interesting feature being that the vasicentric 
parenchyma extends completely around the stem after a wound, while the 
multiseriate rays are confined to the immediate vicinity of the injury. 
Among other poplars with terminal parenchyma and uniseriate rays 
are P . grandidentata and P. laurifolia. On investigation of the latter it 
was found that a very slight wound in the second annual ring is sufficient 
to recall the parenchyma to a moderate extent, while a severe wound further 
out brings back abundant vasicentric parenchyma and multiseriate rays. 
P. deltoides presents a new condition. In this species the parenchyma 
is normally around the vessels, but the rays are uniseriate like those species 
already described. In the first two annual rings of a seedling the rays are 
practically all biseriate, dwindling thereafter to uniseriate, while vasicentric 
parenchyma in that region is extremely abundant. Slightly higher in the 
stem, where the leaf-traces go off, there is a comparatively large amount of 
parenchymatous tissue above and below each trace. A transverse section 
slightly above a trace shows a ray ten to twelve cells wide, which runs out 
for a short distance and then is dissected into multiseriate rays three to 
four cells wide. The other rays throughout the periphery of the stem are 
bi- and triseriate. Abundant vasicentric parenchyma is present in the 
first two annual rings, growing less so in the succeeding years of growth, 
but a transverse section in the plane of an outgoing trace shows that on 
