AN ENCYCLOPEDIA OF HORTICULTURE. 



463 



Oak Galls cuitiinued. 



either by lower plants (Fungi), or by animals, which pro- 

 duce the Galls either for the protection of their young 

 and immature larvae, or for their own protection at all 

 stages of development. The makers of Oak Galls belong 

 almost entirely to the great group of Hymenoptera, known 

 as Cynipidce, or True Gall Flies, almost the only excep- 

 tions being a few Midges, which merely fold down the 

 lobes of leaves, making them become fleshy. 



The Cynipidce are all of small size, seldom exceeding 

 Jin. in length, with a rather greater spread of wing. 

 They vary a good deal in colour from reddish-brown 

 to black and frequently show metallic reflections. The 

 wings are supported by a few nerves. The females pos- 

 sess, at the end of the abdomen, an ovipositor, and, by 

 means of this organ, which is frequently of considerable 

 length, they bore into some parts of the tree, and thereby 

 give rise to the " Gall." 



Galls are found growing from every part of Oak-trees, 

 from roots and branches, to leaves, stamens, and ovaries, 

 or fruits ; and, in harmony with such diversity of situa- 

 tion, they vary exceedingly in their appearance, consist- 

 ency, and internal structure ; but all those of the Cyni- 

 pidce, or Gall i'lies, in the strict sense, to which almost all 

 the Gall-makers in the Oak belong, agree in this, that 

 there is a central space, with walls in which the cells 

 immediately around the space usually contain starch ; 

 while outside this layer there is generally another, for 

 defence, formed of thick-walled, compact cells. In the 

 central cavity lies the single, footless, pale grub of the 

 Gall Fly ; or, if the intruders have destroyed the host 

 larva, two or more larvae of the parasites may be found 

 in each Gall, generally inclosed in separate chairbers. In 

 many of the rounded Galls there is also a considerable 

 layer of beautifully arranged cells (spongy tissue) in the 

 walls. Galls on the roots require to be specially sought 

 for, because of their being concealed under the soil. 



FIG. 705. OAK GALLS. 



A, Gall of Dryoteras terminate (Oak Apple), about two-thirds 

 natural size. 



B, Young Root of Oak, bearing Galls of Biorhiza aptera, about 

 two-thirds natural size. These Galls vary much in size, and are 

 often crowded into a mass, and fused together. When fresh, 

 they are red, and rather fleshy ; but, in drying, they become 

 dark brown and hard. They are believed to be the autumn 

 form of the cycle to which "Oak Apples" belong as the 

 summer form. The insects that emerge from them are all 

 wingless females. 



The Gall of Biorhiza aptera (see Fig. 705) is one of the 

 commonest. The others do not call for further notice 

 here. Galls on the main- stems are not very numerous. 

 Far more conspicuous and important are those formed 

 on the branches. Many of these are modified buds ; 

 numerous others are conspicuous Galls formed upon the 

 leaves, usually on their lower surface ; and still others 

 are stamens or ovaries, changed in structure by the 



Oak Galls continued. 



punctures of the parent insects. On the Continent of 

 Europe, nearly 100 different forms of Galls have been 

 found on Oaks ; and in Britain, the number of forms 

 already observed reaches nearly forty. Among the 

 more widely-known Galls may be mentioned the Oak 

 Apple (see Fig. 705), found, in May and June, on 

 the twigs, as an oblong mass, from lin. to 2in. 

 long, covered with a smooth, green and red skin, above 

 a layer of soft tissue, in which are imbedded numerous 

 larvae, each one in a hard-walled cell or chamber in 

 the centre of the Gall. The Cotton Gall resembles a 

 ball of white cotton wadding surrounding a number of 

 minute thin -walled chambers, each occupied by one 

 larva of small size. The whole mass may reach a size 

 of lin. or more across, and is very conspicuous, as it 

 hangs attached to the male catkins in May and June. 

 The Devonshire, or Marble, Gall of the Oak is very 

 easy to recognise, as it is about the size and form of a 

 boy's marble, green and soft when young, but brown 

 and hard when matnre. Two or more of the Galls often 

 join together during growth, and form irregular masses. 

 In the centre is a small cavity, inhabited by the larva. 

 It is said that this Gall was introduced from the Con- 

 tinent, and was first observed in Devonshire; but, what- 

 ever its origin, it is now abundant in many parts of 

 Britain. Experiments have not succeeded in rearing 

 males, but have yielded many female Gall insects from 

 these Galls, and, along with them, seventy or more kinds 

 of insects that live in the Galls, either as gnests 

 (inquilines) or as true parasites. The name of the Gall- 

 maker is Cynips Kollari. The Artichoke Gall, formed 

 by Aphilothriie gemmae, very much resembles a miniature 

 Artichoke, abont lin. long, formed of scales, with a central 

 chamber like a small acorn. Various kinds of bud Galls 

 are too small to be found without careful search, and 

 are of interest chiefly because of the relation of some 

 of them to other conspicuous forms that disfigure young 

 Oak-trees in pleasure grounds. 



The leaves bear many curious forms of Galls, some 

 of them so conspicuous that they cannot have escaped 

 the notice of even the least observant. One that causes 



FIG. 706. OAK GALLS. 



A, 1, Cluster of Galls of Andricus curvator in Oak Leaves, about 



natural size ; 2. Gall in Transverse Section, showing small 

 inner Gall. 



B, Oak Twig, with two Galls of Aphilothrix collaris, natural size. 



much damage is the work of Andricus curvator (see 

 Fig. 706). It is produced on the midrib, or leafstalk, 

 and causes a marked bend, or twist, in the leaf, or 

 at its base in the branch; the Gall forms a green 

 swelling, about the size of a pea. Inside this lies a 

 small, brown, kidney-shaped inner Gall, which, when 

 ripe, is quite free from the inside of the wall of the 

 large chamber. The Currant Galls, resembling trans- 

 lucent currants, pale, with purple or red mottlings, 



