COMPOUND GALLS IN FLOWERS. 549 



is not the case when the plants are free from the mites; the foliage-leaves in the 

 neighbourhood of the raceme are also lobed and deeply indented, which again is 

 not the case in uninfected plants of this species. In the capitula of the above- 

 named Milfoils the peripheral ray-florets as well as the central tubular ones become 

 leaf-like, and this gives rise to the most peculiar forms. A capitulum is often sepa- 

 rated into several stalked sub-capitula, the flowers being metamorphosed into green 

 funnels with jagged mouths, and into small flat-lobed and toothed foliage-leaves, 

 wMst short, green, scale-like leaflets rise from the midribs of these leaves repre- 

 senting the metamorphosed stamens. A very remarkable "doubling" produced by 

 gall-gnats is to be observed in flowers of the Alpine Rose {Rhododendron ferru- 

 gvMum). The stamens and carpels are changed into red petals by their influence. 

 Since Rhododendron flowers have ten stamens and five carpels, there should be 

 only fifteen red leaflets in the centre of each, but as a matter of fact there are 

 double and treble as many, and there has been not merely a metamorphosis but 

 also a multiplication of the leaves. The flowers of some plants which belong to 

 the Valerians, especially of the Corn-salad (Valerianella carinata), of which a 

 small umbellate cyme is shown in fig. 358 ^, p. 523, are doubled by the influence of a 

 gaU-mite, but without any multiplication of the petals. The doubling is restricted 

 to the transformation of the stamens into a whorl of petals. But we also have 

 another peculiar alteration. The petals become enlarged to more than fifty times 

 their normal size, and change into fleshy lobes which are fused with one another 

 into a disc. As aU these lobes bend back, and become concave below, cavities 

 are formed under the flowers in which the gall-mites can dwell (see fig. 358^, 

 p. 523). 



The axis of the inflorescence and the stalks of single flowers are often thickened 

 and fleshy in these cluster-galls, and they are also stunted and bent in the most 

 varied manner. If several neighbouring pedicels fuse together, structures like 

 cocks'-combs, or like a compressed and flattened branch, arise; to these the term 

 fasciation is applied. Sometimes when numerous pedicels arranged in the form of 

 umbels fuse together we have structures like coral-colonies, or irregular clumps 

 which are beset with green flowers usually much reduced in size. This is the case 

 in the fasciations of the Ash (Fraxinus excelsior and Ornus), which are caused by 

 a gall-mite (Phytoptus), and which occur so abundantly that it looks as if the tops 

 of the trees had been sown with them. 



The enumeration, here, of various forms of galls commenced with the incon- 

 spicuous felt-galls on the under side of isolated foliage-leaves, and it ends with the 

 complex cluster-gaU, in which hundreds of flower-stalks and leaves are frequently 

 concerned. Of course, only types of the individual groups which have been mentioned 

 in this long series could be instanced, and we cannot make any attempt to describe 

 all the gall-structures at present known, about 1600 in number. Whether the 

 extension of gall-researches in tropical regions will yield new forms which stand 

 outside the pale of the classification given it is difiicult to say. Apparently this will 

 not be the case. Perhaps thousands of hitherto unknown galls might be added 



