Stem Gall on Muehlenbeckia australis — Arnold 
493 
of a species of gall midge, they failed to re- 
enter new shoots of the host plant.) With the 
larvae of Morova subfasciata, however, further 
work with intact seedlings, rather than cuttings, 
of M. australis might give promising results 
under laboratory or glasshouse conditions. 
Such experiments depending on the feeding 
action of the larvae may be more fruitful than 
attempts to induce galls with larval extracts. 
In the experiments in which crude acetone ex- 
tracts of larvae were applied to seedlings of 
M. australis, there was no sign of growth 
stimulation in the treated plants after 18 
months, at which time the 48 seedlings were 
discarded. 
Sustenance for the larva is provided by the 
continued growth of the pith cells of the gall. 
When living galls are cut open and kept moist 
in a petri dish, after removal of the larva, 
white callus-like proliferations of the pith be- 
come apparent to the naked eye after a week 
at room temperature. (If the larva is left 
inside a gall from which only a "window” of 
tissue has been removed, the larva forms a 
smooth dark membrane resealing the cavity in 
a few hours.) No callus-like proliferations are 
obtained from living galls from which the moth 
has departed; and it appears that the larva 
eventually consumes the entire pith tissue with- 
in the length of the gall. 
A comparison of the anatomy of the gall 
with that of normal stems of M. australis shows 
some interesting differences which seem to 
account for the process of gall enlargement. 
Serial cross sections through young galls and 
the adjacent portions of the stem show that the 
presence of the larva is associated with exces- 
sive growth of vascular rays and pith. The pith 
and vascular rays enlarge and actively invade 
the woody cylinder (Fig. 4), pushing outward 
to the cortex. At the same time, or shortly after- 
wards, the cambium itself becomes overactive, 
giving rise to further ray tissue of irregular 
starch-packed parenchyma. The end result of 
this excessive growth of pith, rays, and cam- 
bium is that the vascular cylinder is cleft into 
several cable-like strands (Fig. 5) which tra- 
verse the body of the gall like a cage around 
the larva. 
In older normal stems of M. australis the 
vascular cylinder resembles a scalloped column. 
Presumably this is the result of the cambium 
forming larger proportions of xylem than of 
phloem at the semicircular xylem lobes, and a 
corresponding excess of phloem in the V-shaped 
grooves between the lobes (Eames and Mc- 
Daniels, 1925). This atypical growth of the 
cambium is associated with the laying down of 
conspicuous rays which are continuous in spoke- 
like fashion (in cross section) from the pith 
to the grooves between the xylem lobes. 
Nevertheless, the normal stem of M. australis 
is a compact unity and not a composite of 
separated strands like the body of the gall or 
the typical stems of other lianes. It is well 
known that in several lianes anomalous activity 
of the cambium may give rise to a separation 
of the vascular system into ropelike strands 
(Eames and McDaniels, 1925). 
Thus, the morphogenetic influence of the 
moth has been to initiate abnormally high 
growth rates and new growth patterns in the 
pith, rays, and cambium; and to produce the 
counterpart of advanced liane-type anatomy in 
a plant which by itself has not evolved far in 
this direction. 
DISCUSSION 
The tendency of the cambium of the normal 
stem of M. australis to produce conspicuous 
vascular rays and varying proportions of xylem 
and phloem at different sites could be taken 
as the suggestion of an evolutionary trend 
toward the type of stem structure seen in lianes 
with a vascular system composed of separated 
strands. Without this slight tendency being al- 
ready inherent in M. australis, it is doubtful 
whether the larva of the gall moth would elicit 
such a dramatic anatomical transformation. 
The persistence of the galls as an overgrowth 
after the departure of the insect may reflect 
merely the continued growth of tissues after a 
preliminary boost and reorientation, without in- 
dicating a truly tumourous condition, and may 
not be entirely contradictory to the concept of 
tumour growth implied in the statement of 
Brues (1946) quoted earlier. The initial inva- 
siveness of pith and ray cells may reflect the 
stimulatory effect of wound hormones resulting 
