GALLS 177 



in most parts of the entire plant, which may be greatly 

 altered. Indeed, so great is the change in some cases that 

 the affected plants have been taken for species distinct from 

 the type. Such conditions, which scarcely fall under 

 the usual idea of galls, are generally the work of fungi. 

 \Yidely extended effects are also produced by some of the 

 gall-mites when they give rise to the condition known as 

 virescence, the inflorescences being replaced by small ill- 

 formed green bodies, and the foliage leaves being diminished 

 in size, and usually more or less cut into narrow divisions or 

 fringes. But in most cases the part attacked is strictly 

 limited, though some species gall one organ at one period, 

 and a different one at another, e.g. the leaf and root-galls of 

 Phylloxera vastatrix on the Vine. The two forms of galls 

 formed by the dimorphic generations of some gall insects 

 have been already referred to. These, in their highest 

 development, are confined in Britain to Oaks. 



A gall-maker does not often affect plants of more than 

 one natural order, except those that show a well-marked 

 alternation of hosts, as do a good many fungi and some 

 Aphides. These insects alternate between Dicotyledonous 

 trees or shrubs, less often herbs, and grasses, on which they 

 pass the winter. There is evidence that tends to show that 

 some of the gall-mites also can live on, and distort plants of, 

 different orders. 



The various forms of galls have been ranged, as regards 

 their structure, under two groups (i) Simple, in which one 

 member alone is affected, and (2) Compound, or Bud-galls, in 

 which the shoot and the members that it bears are more or 

 less altered. 



The Simple galls have been divided into the subjoined 

 groups : 



A. /*>//-galls, better called Hair-galls, composed wholly, 

 or at least chiefly, of hairs on the surface of the part affected. 



B. Mantfe-galls, formed by the part curving round, or by 

 a pouch being formed by the increased growth of a limited 

 area, resembling the finger of a glove projecting from the 

 surface, or a pit formed by the more rapid growth of the 

 sides of the cup. However formed, the hollow surface is 

 lined with epiderm, corresponding to that of the outer sur- 



