892 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
rounding parenchyma. A similar filling of the canals with parenchyma has once or 
twice been observed in the ordinary free stems. 
The primary xylem-bundles of the branch are usually surrounded on the outside by 
a zone of radially-arranged secondary wood, no doubt the product of the cambium of 
the branch. In fig. 21 we see that secondary wood has only been formed towards the 
upper side of the branch. On the lower side the primary wood was continuous with 
that of the stem, and so no cambium could be formed. Further towards the exterior 
this hindrance no longer exists, and the zone of secondary wood becomes complete 
(see photographs 5 and 6, Plate 72). 
The secondary wood of the branch is connected in the most complete manner with 
that of the main stem on which it is inserted. The direct connection is mainly from 
below. Here the secondary tracheides of the parent axis abut immediately on those 
of the branch. Other tracheides pass up from below, at the sides of the branch, and 
bend over to join its lateral strands of xylem, while others again curve round over the 
top, and bend down to form a union with the wood on the upper side of the branch. 
The whole structure bears a most striking resemblance to the insertion of a lateral 
shoot of Pinus, as figured, in tangential section, by Professor STRASBURGER.* Occa- 
sionally, a few tracheides appear to be continuous in the upward direction, from the 
branch into the wood. 
A radial section through the insertion of a branch, figured in a previous memoir,t 
shows with great clearness how the pith of the branch terminates inwards in a narrow 
neck, by which it is continuous with the pith of the parent stem. The same fact is 
shown in various transverse sections (see Plate 79, fig. 20, which, however, is not 
median through the branch ; and Plate 80, fig. 22). ‘The series of transverse sections 
cut from the base of the specimen discovered by Mr. Wixp, and already referred to, 
also illustrates the point (see Plate 73, photographs 7, 8, and 9). At a distance 
from its base the branch has a large fistular pith, surrounded by a ring of about 
24 bundles, of the normal structure (photograph 7). Lower down the pith is much 
smaller ; the bundles are still normal, but are much reduced in number (photograph 8, 
which shows 14 bundles). At the base itself the pith is reduced to a minute body, 
and is no longer fistular. The vascular bundles are not more than 10 in number, 
are crowded closely together, and their canals can no longer be recognized (photo- 
gragh 9). In fact, we have here almost reached the inner termination of the branch, 
such as is shown, in connection with the main axis, in Plate 80, fig. 21. All these 
facts agree exactly with the appearances presented by the medullary casts (see 
Plate 86). 
So far as we have gone, the structure of the branches in their basal regions has 
been sufficiently intelligible. Certain difficulties, however, remain. If we examine 
tangential sections through the outer layers of the secondary wood, and passing 
* ‘Histologische Beitriige,’ vol. 3, plate 2, fig. 40. 
+ Winttamson, “ Organization,” Part IX., Plate 21, fig. 27 (C.N. 97). 
