84 The Natural Abscission of Twigs. 
series of vessel elements in the swollen mass of parenchymatous 
tissues) . 
A curious feature of the parenchymatous zone is the sharp 
delimitation from the lignified tissues above and below. It is 
probably located in the bud scale region and may possibly 
be related to the type of vascular structure initiated in the 
axis from these structures. 
In the typical internodal region of branch or axis the 
wood consists of vessels and fibres, with a small proportion 
of wood parenchyma at the face of the summer wood and 
around the spring vessels. The rays are usually only one 
cell in width, and the spring and summer wood are sufficiently 
unlike one another for it to be possible to distinguish the 
annual rings, though these are relatively inconspicuous in this 
species. In the parenchymatous zone, on the other hand, the 
vessels are very few in number and are not accompanied by 
typical lignified fibres. The pitting on the vessel walls also 
differs from the usual ‘ pitted ' types and approximates to 
scalariform. The wood parenchyma is considerable in amount 
and the rays are several cells in width. The fibres of the 
typical wood appear to have been replaced by hies of 
parenchymatous cells. All the living cells in this zone 
appear to be less differentiated than in normal wood and many 
of the parenchyma cells and ray cells appear to be practically 
un vacuolated. 
The differentiation of the phloem undergoes comparable 
modifications in this zone. The sieve tubes and fibres of the 
typical phloem are substituted almost entirely by parenchyma. 
The cells of the cambium are shorter than those of the 
internodal region, being only about J-§ the typical length. 
The cambial ring is also much interrupted by the numerous 
initials of the expanded rays. 
Discussion. — It is now firmly established that cambial 
activity, followed by vascular differentiation, is initiated in 
the opening buds and extends basipetally down the shoot 
(3 and 4). The vigour of activity extending down the shoot 
varies with the vigour of the bud undergoing extension. It 
is interesting from this point of view to find that in P. serotina 
the abscissed twigs are the weak vegetative and the 
reproductive. The former are usually weak from the outset, 
being derived from small buds and usually opening late when 
they make little headway against the competition of the 
other growing shoots. Further, these branches are usually 
in such a position that they are likely to be developing under 
conditions of poor light. Thus the conditions are all adverse 
to a vigorous vascular differentiation. In the bud scale 
region the primary structure is evidently not conducive to a 
transmission of typical vascular differentiation, and although 
The Naturalist 
