528 ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



growths, like small rosy-cheeked apples, which occur on the foliage of Oaks 

 popularly known as " oak-apples ", are amongst the best known. The terras 

 " gall " and " gall-apple " were used by writers in the sixteenth century, and (like 

 the Old English word galle, the French galle, and the Italian galla) are derived 

 from the Latin word galla, used for these outgrowths by Pliny in his Natural 

 History. The sixteenth-century writers distinguish between "gall-nuts" and 

 " gall-apples ", meaning by the former the small hard outgrowths on the leaves 

 of Beech-trees. Afterwards the word gall was used for all the outgrowths 

 produced by animals on green living plants. More than that — the hypertrophies 

 described in the preceding chapter, produced in green host-plants by the various 

 families of Fungi, are also included under the term. It has been proposed recently 

 to substitute the word cecidium for gall, and to distinguish the excrescences as 

 myco-cecidia, nemato-cecidia, phyto-cecidia, diptero-cecidia, &c., according as they 

 owe their origin to Fungi, Thread-worms (Nematodes), Gall-mites (Phytoptus), 

 Gnats (Diptera), &c. A systematic classification of this sort, on the lines of 

 the classification of animals, might be of use to Zoologists, but to the Botanist its 

 value is only secondary. He must, as in other similar cases, keep to morphology as 

 the primary ground of classification, and has to arrange the structures according 

 to their agreement in development. Moreover, in a general review, it is necessary 

 to consider whether a whole group of plant-organs or one alone undergoes metamor- 

 phosis; and the starting-point of the outgrowth must also be ascertained; i.e. 

 whether it is the foliage-leaves, floral -leaves, stems, or root -structures, &c., which 

 are the head- quarters of the excrescence. 



When the gall originating as the nest or temporary habitation of a single 

 animal or colony of animals is limited to a single plant organ it is said to be 

 simple; if, on the other hand, several plant organs are concerned in its production 

 it is said to be compound. 



8im,ple galls may, for convenience of description, be divided into (1) Felt- 

 galls, (2) Mantle-galls, and (3) Solid galls. The Felt-galls are chiefly due to 

 hypertrophied epidermal cells growing out into hairy coverings of various sorts 

 and shapes; Mantle and Solid galls, however, are rather more complicated. 

 In both cases insects are present in swellings of various descriptions, but there 

 is this essential distinction : — The Mantle-gall is a hollow structure which, 

 though it may arise in various ways and assume a multiplicity of forms, always 

 has a portion of the surface of the affected organ for its lining — in other words, 

 it is a chamber formed by hypertrophied growth around the place occupied 

 by the insect. In the Solid gall, on the other hand, some spot is pierced by an 

 insect and the eggs deposited in the tissues (not on the surface), the punctured 

 spot forms a swelling with the larva inside, but the lining of the chamber is 

 in no sense a portion or development of the original surface of the organ afiected. 

 Again, whilst in most mantle-galls the cavity of the gall is in open communication 

 with the outside, and the insect can escape by this aperture (though this is not 

 invariably the case), in the solid gall there is not such opening, and the msect 



