AN ENCYCLOPADIA 
OF HORTICULTURE. 
Oak Gails—continued. 
entomologists, pre-eminent among whom stands Dr. Adler. 
The following are two very noteworthy peculiarities con- 
nected with many Oak Galls: (1) From certain kinds, 
e.g., Devonshire Gall, Oak Spangles, &c., females alone 
have been reared (sometimes in thousands), while from 
others males as well as females are readily obtained 
in confinement. Where females alone emerge from Galls, 
they Jay eggs that produce larve, though unimpregnated. 
But we can hardly imagine that this mode of repro- 
duction can go on perpetually without requiring to be 
re-invigorated by the influence of males. (2) Certain 
kinds of Galls appear only in early summer; e.g., Currant 
Gall, Blister Gall, and Oak Apple. The insects emerge from 
the Galls in June or July, and may be observed to lay 
their eggs at once on the twigs, buds, or leaves; yet 
a 
no trace of the Galls can be seen before the succeeding 
spring. In like manner, many Galls are met with only in 
autumn, e.g., Oak Spangles, Silky Button Galls, &c., 
and the insects emerge from them in spring, lay eggs 
on the Oaks, and disappear; but the Galls re-appear only 
in autumn. 
The insects reared from each form of Oak Gall are 
distinguishable, by an expert, from those reared from 
every other form, the differences affecting form and size 
of the ovipositor, wings, and other important organs, as 
well as such minor points as colour. Formerly, it was 
very difficult to understand alike the constancy with 
which only females emerged from some Galls, and the 
(supposed) length of time between the puncture of the 
plant by the parent Gall Fly, and its result in the forma- 
tion of a Gall, some months later. The explanation of 
both difficulties now seems afforded to us in the dis- 
covery, first announced from the United States by Mr. 
Walsh, in 1870, but chiefly wrought out and enforced 
in its application to the life-history of Gall Flies (Cyni- 
_ sufficiently explains the pro 
pide) by Dr. Adler. This explanation is, that most of 
these ‘insects appear in two annual broods; that these 
broods differ from one another in the Galls formed by 
them, as well as in the structural characters of the mature 
insects; in short, that the insects and their Galls alike 
are dimorphic—a unisexual brood appearing in winter 
or spring, laying eggs, though unimpregnated, and pro- 
ducing Galls in early summer, from which emerge, in 
a month or two, insects differing so much from their 
parents (makers of the Galls) that they have been re- 
ferred to genera different from the latter. The later brood 
possess males as well as females: these pair, and the 
females produce Galls similar to those with which the cycle 
was begun. Such a cycle has now been traced in many 
of our native Gall Flies. Subjoined are noted only five 
such cycles, all figured above, since even to enumerate 
all the cases would exceed the space available here. The 
two forms of Galls are, in each case, placed in the same 
line, the spring or early summer form to the left, the 
autumn form to the right. They are as follows: 
Aphilothria collaris. 
Biorhiza aptera. 
Neuroterus lenticularis. 
Dryophanta fobii. 
Neuroterus numismatis. 
Andricus curvator. 
Dryoteras terminalis. 
Spathegaster baccarum. 
Spathegaster Taschenbergi. 
Spathegaster vesicatriæ. 
It is at once evident that, if this theory of the exis- 
tence of dimorphism among Oak Gall Flies be true, it 
blems, stated above, in regard 
to unisexual insects alone being reared frequently, and 
to the supposed interval between the puncture and the 
formation of the Gall. : 
Remedies. It is seldom necessary to take active 
means to reduce the numbers of Oak Galls, although, at 
times, the lower surface of the leaves— almost every 
square line upon it— bears one or more Galls, such as 
Oak Spangles and others of the smaller kinds. Their 
presence does not seem to weaken the trees very mate- 
Vol. II. 
Oak Galls—continued. 
rially—a fortunate circumstance, since remedies could 
scarcely be applied with success to huge trees, or even 
to Oak-bushes. Almost the only Gall deserving to be 
mentioned as distorting twigs and leaves, is the work of 
Andricus curvator, The supposed second stage of this Gall 
is a tiny egg-shaped excrescence, half hidden among the 
leaf scales in buds; the insect reared from it has been 
named Aphilothriæ collaris. Where so many occur as to 
injure the plants, the best remedies are to hand-pick 
and to destroy the Galls. Another Gall that occasionally 
distorts the branches of Oaks, though much less often 
than the last-mentioned kind, is that of Andricus 
inflator: it is a swelling at the tips of the young twigs. 
In the centre is a cup-shaped hollow, closed above by a 
thin membrane; and at the bottom of the hollow is a 
small, ovate, brown, very thin inner Gall. The insects 
emerge in July, and there are both males and females 
amongst them. The Gall believed to alternate with this 
is globular, scarcely over gin. across, green, and smooth; 
and is formed in the buds in autumn, remaining concealed 
in the bud scales. The insects emerge in early spring, are 
all females, and have been named Aphilothrix globuli. 
OAT, or OATS. See Avena. 
OB. This term, used in the composition of Latin 
technical terms, signifies inversion; e.g., obovate means 
inversely ovate ; oblanceolate, inversely lanceolate, &c. 
OBCOMPRESSED. Compressed, so that the two 
sutures of a fruit are brought into contact; flattened, 
back and front. 
OBCONICAL. Inversely conical. 
Fic, 710. OBCORDATE AND RETUSE LEAP, 
OBCORDATE. Inversely cordate. An Obcordate 
and retuse leaf is shown at Fig. 710. 
OBELISCARIA. Included under Rudbeckia (which 
see). : 
OBERONIA (so called after Oberon, the Fairy 
King, in allusion to the quaint and variable forms. of 
the plant). ORD. Orchidee. A genus comprising about 
fifty species of stove, epiphytal, tufted orchids, natives 
of tropical Asia, Australia, and the Mascarene and 
Pacific Islands. Flowers small, in numerous dense, sub- 
cylindrical racemes, or spikes, sessile, or shortly pedi- 
cellate; sepals free, sub-equal; petals narrower, or 
shorter, than the sepals; lip sessile, usually concave. 
Leaves distichous; sheath compressed, equitant. The 
species are of botanical more than horticultural interest. 
Those described below will grow attached to blocks of 
wood, suspended near the glass, in a warm, moist 
house. Care must be taken to guard against superfluous 
moisture, both at the root and in the air, especially 
during winter. Propagation is effected by dividing the 
tufts. 
„ acaulis (stemless-leaved). fl. o. e, numerous, on a long, 
„ 3 sepals and 3 uniform, . y 
reflexed; lip sub- q ar, three-lobed, the middle lobe bi- 
lobed; column very short. Spring. J. stemless, few, the longest 
lft. long, ensiform, falcately recurved, long-acuminate. Eastern 
Bengal. (B. M. 5056.) 
O. iridifolia (Iris-leaved). A synonym of O. tahitensis, 
0. (Otaheite). fl. yellow, very numerous, verticil- 
; reflexed, ovate, equal; petals reflexed, erose; lip 
erect, with a fimbriated, involute margin. June. J, ensiform, 
similar to those of an Iris, drooping or pendulous, sometimes 
longer, sometimes shorter, than the raceme. Pacific Islands, 
1840 (B. M. 4517, under the name of O. iridifolia.) 
3 0 
