SCIENCE-GOSSIP. 



121 



of the leaf, leaving always a distinct mark. Months 

 pass before any gall- formation can be seen, it is 

 not till September that the egg hatches out, and 

 the delicate mandibles of the larva start the active 

 cambium cells into gall-formations. A gall is not 

 parasitic in the surrounding tissue, it is of the same 

 elements, only substituting itself for them by faster 

 growth and still growing proportionately to the 

 growth of the cellular layer around it. In a leaf- 

 gall the formation begins in the layer of formative 

 cells on the under surface. Those of the upper 

 surface havmg already become stable, they can 

 undergo no further change, and therefore respond 

 to no irritation ; they are incapable of forming new 

 cells. At first the cell-growth only affects a small 

 zone around it, but as it acquires a vascular system 

 of its own it begins to grow as an independent 

 structure. It is different when the eggs are laid in 

 a bud, then the biting larva touches rudimentary 

 leaves consisting of still unmodified cells, all equally 

 capable of development whether of upper or under 

 surfaces. Then both surfaces take part in gall 

 formation, and when the leaf comes to be unfolded 

 it is found that there is an absence of leaf tissue, 

 and that the resulting gall grows through the leaf 

 substance. 



Again, it is different when eggs are laid in the 

 cambium layer of the bark. Here the cells which 

 first form round the larva, cannot be distinguished 

 from adjacent cambium tissue, but in later growth 

 there is a great contrast. The outer zone of the 

 cambium ring produces the cells of the bast 

 parenchyma, while the central zone of the cambium 

 produces the wood parenchyma, and in these galls 

 there is, too, a soft zone of sappy parenchymatous 

 cells, and a hard central zone of wood parenchyma. 

 In all bark-galls the woody centre penetrates into 

 the woody tissue of the tree, while the soft fleshy 

 circumference proceeds from the bark. New cell- 

 growth is arranged in concentric layers round the 

 larva, accompanied by changes in cell contents. 

 The cells next the larva swell out, the cell contents 

 become cloudy, and a multitude of starch granules 

 appear. The rudimentary gall draws its first 

 nourishment from the surrounding tissue, later it is 

 more independent, for a new element comes in. 

 From the spiral vessels lying in the cambium ring, 

 processes are driven into the rudimentary gall ; 

 the entrance of these vessels occurs at a definite 

 spot on the lower surface of the gall, whether it is 

 connected with the parent tissue by a broad base 

 or a small stalk. The gall has now become an 

 independent structure and is practically withdrawn 

 from the direct influence of the cellular area 

 around it, from which it sprang. Its individuality 

 of organisation is shown by complicated transmu- 

 tations of cells originally alike, especially in the 

 cells of the exterior, which develop peculiar 

 pigments and hairs of various kinds, both in great 



variety of forms. It is the value of these different 

 structures, as protective contrivances, which has 

 secured their evolution by the gall. Some- 

 times the hairs exude a sticky sap which 

 keeps off parasites. Even smooth galls, like 

 Aphiloterix sieboldi, secrete a juice which attracts 

 ants, these protect the galls, like sentinels, driving 

 other insects off and often constructing a protective 

 mantle of earth around them. If the larva 

 perishes before the gall is mature its formation 

 is stunted. The influence of the larva is necessary 

 not only for the commencement but for the 

 completion of the gall. When a roundish inner 

 gall is found undeveloped, parasites are always 

 present. A gall pricked by parasites grows in an 

 anomalous manner. Galls contain not only the 

 larvae that form them, they are often taken 

 possession of by insects that are called " inquilines " 

 or lodgers of the oak-gall flies. These creatures 

 enhance the natural difficulties of observation of 

 gall-formation ; they are so nearly related to the 

 true gall-flies that they can only be distinguished 

 by the minutest characteristics. It is not doubted 

 that they have been developed from the true gall- 

 flies. By the use of a gall already formed the 

 prosperity of their progeny is more certainly 

 ensured. Unfortunately, these lodger-flies are 

 more easily reared and collected than the true gall- 

 makers. The gall-fly proceeds with great care in 

 the choice of tender leaves, or terminal buds, or 

 flower buds, but in spite of its care galls often 

 fail to appear where eggs have been laid. 

 The greatest number fail in the buds where 

 only one egg is laid. Species emerging in 

 summer can only prick winter buds which 

 are waiting the coming of the next growing 

 period, and in many seasons a premature and 

 anomalous development of winter buds may be 

 absent. This is not the only reason ; the egg must 

 be placed exactly in the cambium ring, which lies 

 like a fine seam in the base of the bud, and if the 

 egg is not laid in this fine seam, it perishes without 

 forming a gall. Considering the difficulty to be 

 overcome in placing the egg in precisely the right 

 spot, it is not surprising if many eggs are laid 

 amiss. Failures occur less frequently in leaf-galls 

 pricked in bud, because the fly has choice of much 

 wider territory — the whole of the rudimentary leaves 

 in the bud. Failures are not usually observed at 

 all where the fly pricks the surface of bark or leaf, 

 because the cell region to be struck is always at one 

 uniform depth below the surface. Gall-formation 

 is dependent on the growing period of the tree, and 

 ceases at its close. Most galls mature in the space 

 of a year. Those which require two years are bark- 

 galls ; the first year the rudiment is formed and then 

 development ceases till the next spring, when it is 

 resumed with the new period of vegetative activity. 

 Dr. Adler's book is beautifullv illustrated : all 



