AN ENCYCLOPEDIA OF HORTICULTURE. 



465 



Oak Galls 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 lay eggs that produce larvae, 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 

 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- 

 pidce) 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: 



Andricus curvator. Apliilothrix collaris. 



Dryoteras terminalis. Biorhiza aptera. 



Spathegaster baccarum. Neuroterus lenticularis. 



Spathegaster Taschenbergi. Dryophanta folii. 



Spathegaster vesicatrix. Neuroterus numismatis. 



It is at once evident that, if this theory of the exis- 

 tence of dimorphism among Oak Gall Flies be true, it 

 sufficiently explains the problems, 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. IL 



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 Aphilothrim 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 fin. 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 Aphilothrir- globuli. 



OAT, or OATS. 



Aveiia. 



OB. This term, used in the composition of Latin 

 technical terms, signifies inversion ; e.g., obovate means 

 inversely ovate ; oblanceolate, inversely lanceolate, &c. 



DECOMPRESSED. Compressed, so that the two 

 sutures of a fruit are brought into contact ; flattened, 

 back and front. 



OBCONICAIi. Inversely conical. 



FIG. 710. OBCOIIDATE AND RETUSE LEAP. 



OBCORDATE. Inversely cordate. An Obcordate 

 and retuse leaf is shown at Fig. 710. 



OBELISCAB.IA. Included under Rudbeckia (which 



see). 



OBERONIA (so called after Oberon, the Fairy 

 King, in allusion to the quaint and variable forms of 

 the plant). OBD. Orchideae. 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. 



O. acaulis (stemless-leaved). fl. orange, numerous, on a long, 

 cylindrical raceme ; sepals and petals uniform, ovate, singularly 

 reflexed ; lip sub-quadrangular, three-lobed, the middle lobe bi- 

 lobed ; column very short. Spring. 1. stemless, few, the longest 

 1ft. long, ensiform. falcately recurved, long-acuminate. Eastern 

 Bengal. (B. M. 5056.) 



O. iridifolia (Iris-leaved). A synonym of 0. tahitensis. 



O. tahitensis (Otaheite). /. yellow, very numerous, verticil- 

 late ; sepals reflexed, ovate, equal ; petals reflexed, erose ; lip 

 erect, with a timbriated, involute margin. June. I. 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 0. iridifolia.) 



3 o 



