ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 527 



colour. The Bearberry {Arctostaphylos Uva-ursi), Ledum palustre, and the Marsh 

 Andromeda (Andromeda polifolia) are subject to similar metamorphoses at the 

 hands of Exohasidium Vaccinii, so that Vaccinium Vitis-Idcea may be regarded 

 as typical of them. 



When the shoots of the larger shrubs or trees are metamorphosed by parasitic 

 Fungi attacking their branches, we have the formation of the structures popularly 

 termed Witches' brooms, which were mentioned at the beginning of this chapter. 

 The stimulus necessary for their formation is afforded in different plants by 

 different parasites; on Barberry bushes (Berberis vulgaris) by jEcidium Magel- 

 hcanicum (to be distinguished from the common ^. herberidis), on the Gray 

 Alder (Alnus incana) by Exoascus epiphyllus, on the Hornbeam (Carpinua 

 Betulus) by Exoascus Carpini, on the Bullace (Prunus insititia) by Exoascus 

 insititice, on other species of the genus Prunus by Exoascus Cerasi, on the 

 Birch (Betula verrucosa) by Exoascus turgidus, on the Weymouth Pine (Pinus 

 Strobus) by Peridermium Strobi, and on the Silver Fir (Abies pectinata) by 

 ^cidium elatinum. Witches' brooms also occur on the Mastic tree (Pistacia 

 Lentiscus), and on Beeches, Pines, Larches, Spruce Firs, &c., although hitherto 

 we have not been able to ascertain definitely what parasitic Fungi are the 

 cause in these cases. The Witches' broom of the Silver Fir has been selected 

 and figured (see fig. 359) as a type of these peculiar structures. It always 

 grows on one of the horizontally projecting lateral branches of the Fir, and 

 raises its erect or curved twigs from the upper side, resembling, as it were, an 

 epiphyte growing on the bark of the horizontal bough. The twigs are grouped 

 in whorls and not in two rows, as usually happens in the lateral shoots of the 

 Silver Fir. They are all shortened and thickened, and remarkably soft and 

 pliable, because the cortical parenchyma has become spongy and the wood is 

 only slightly developed. The buds, which in healthy tissue are egg-shaped, 

 are almost spherical here. As in other instances of hypertrophied plant-members, 

 we have a precocious development, a so-called "prolepsis", in these Witches' 

 brooms. The buds swell earlier and unfold earlier than those of healthy twigs. 

 The leaves remain short, yellow, somewhat crumpled, and fall off when a year 

 old, while those of normal twigs are long, linear, straight, dark green on the 

 upper side, and remain in position from 6-8 years. The growth of the twig 

 is restricted; it dies off in a few years, and then, inserted on the dark green 

 branches of the Silver Fir, remain the dry, bristling brooms, whose appearance 

 has stimulated the imagination of the peasantry and given rise to the supei-stitions 

 alluded to at the beginning of this chapter. 



ALTERATION OF FORM BY GALL-PRODUCING INSECTS. 



Certain members of the Arachnoidea, Diptera, and Hymenoptera, which 

 attack and penetrate the tissues of living plants and incite the formation of 

 peculiar excrescences, are known as gall-mites, gall-gnats, and gall-wasps. The 



