il ' ii - Northern Research Station - USDA Forest Service

iomass to.gall tissues (fig. 4), mainly at the expense of When the young larvae still feeds on the enwrapped
allocation to the remaining part of the stem (-14 percent; leaves above the growing point, the first indications of
ANCOVA Fi.60 = 131.149, p < 0.001) and the leaf blades the gall formation become visible. The cone-shaped
(-17 percent; Fi,60 109.938, p < 0.001). Allocation to apical meristem is more flattened (fig. 6-1). During
sheaths only decreased very slightly (-3 percent; F1.60= feeding, the larva does not touch the growing point. The
4.257, p= 0.043). marrow tissue expands and the tissue cylinder grows ,,
wider. We found that septa are still present, although
Internal Changes of Galls and Uninfested Shoots their development is strongly reduced (fig. 6-2). They
are confined to a layer of small parenchymatous cells:
A full-gr0w n intemode of an uninfected shoot is hollow. The number of visible septa is also reduced near the
The inner side is 0nly covered by a thin layer of rather growing point (fig. 6-1). Only at the bottom of the gall,
loosely connected empty cells (fig. 5-1). These are the septa consist of a dense layer of small parenchymatous
remains of the central pith which degenerates during cells with some lignified cells.
intemode maturation. In youngest intemodes, right
under the growing point (fig. 5-4), the pith consists of A cross section through a mature L. lucens gall basically
more or less ;rbunded cells with small intercellular gaps. reveals the same sequence of tissue layers as in a
' Lower in the shoot, larger cavities arise between the normally developed stem (fig. 5-6). The central pith
shrinking cells, untilthe pith completely degenerates. At however, is completely filled with a dense mass of a
a node a septum intersects the marrow cavity. This parenchymatous tissue, consisting of large round cells.
septum consists of xylem, phloem, small parenchyma Only at the bottom of t-hegall chamber is the marrow
cells and a few sclerenchyma Cells (fig. 6-3). The actual provided with some air channels.
tissue cylinder of a stem is bound on the outside by a
single layered epidermis (fig. 5-1), followed by the In addition to the differences in the central marrow, the
subepidermal sclerenchyma and the subepidermal surrounding tissue cylinder also changes slightly. The
parenchyma. More centrally we find a second, outer larval chamber is coated with a special layer of sclerensderenchymatous
layer. The inside consists of a thick chyma cells on the inside of the tissue cylinder, not found
layer of transversely rounded to polygonal, longitudinal in an uninfested stem (fig. 5-6). This sclerenchymatous
cylindrical cells, the ground parenchyma. In the ground sheath consists of two distinct layers, each composed of
parenchyma run the collateral vascular bundles, sur- several cell layers. The sclerenchyma cells of the inner
rounded bya perivascular sclerenchyma. At the height layer are orientated longitudinally, parallel to the stem
of the subepidermal parenchyma, the tissue cylinder can axis. These cells are orientated circular around the stem
beprovided with aerenchyma strands. These are axis in the outer layer. This zone is perforated regularly
particularly well developed in submerged shoots and by horizontally orientated vascular bundles which run
rhizomes (Armstrong and Armstrong 1988, Rodewald- around the larval chamber, perpendicular to the normal
Rudescu 1974). orientation. They run from the vascular bundles in the
1.0
• .
- leaf
0:8
,
"o
(1)
0_
_ 0.6
sheath I1
. "_
E
0
Ir
" 1::
0
Q.
o
I,.
Q.
0.4 stem
178
0.2
0.0
ungalled galled
Figure 4._Proportion of biomass allocated to constituent organs in
" galled and ungalled shoots ofPhragmites australis. The largest
standard error was +__O.030for allocation to sheath.
gall