GALLS 175 



more highly specialised galls) often a great diversity in 

 structure, uses, and contents among the constituent cells. 

 In this sense galls may be said to occur on animals as well 

 as on plants ; but the galls on animals are comparatively 

 few, and need not be considered here. Passing to plants as 

 hosts, we meet with galls, necessarily of very simple struc- 

 ture, even among some of the Algae or water-weeds, e.g. in a 

 few red sea-weeds, and in species of the green freshwater 

 weed Vauchcria, which bear flask-shaped galls caused by a 

 Rotifer --Notonnna Werneckii. But the whole number of 

 galls on flowerless plants is very small, even the ferns and 

 their allies bearing but few, and of a structure but little 

 specialised. Many of the flowering plants, on the other 

 hand, show a very wide range of structure and form in the 

 galls they bear. By far the most specialised forms known 

 to us occur on some of the Dicotyledons. It is curious to 

 find that the Gymnosperms show a far less development of 

 galls, alike in structure and in frequency, than do the Angio- 

 sperms, or flowering plants with closed seed-vessels, though 

 they so long preceded the latter plants in their appearance 

 on the earth's surface. Among the Angiosperms, the two 

 great divisions of Monocotyledons and Dicotyledons present 

 a similar inequality ; the former division, though the earlier 

 to appear in the earth's history, being by far behind the 

 other in the complexity of structure reached by the galls, 

 and also in the number of kinds of gall-makers in comparison 

 with the number of species of plants in each division. 

 Certain orders of Dicotyledons are very susceptible to attacks 

 of gall -makers, every species bearing one or more kinds. 

 Pre-eminent alike in number of forms, in high specialisation 

 of many of the galls, and in the prevalence of dimorphism 

 (or two forms of a single species in successive generations 

 within the year producing two forms of galls), stand the 

 Oaks. The other genera (beech, hornbeam, and hazel) of 

 the family of Cupuliferae are also more or less infested by 

 galls. The allied families Betulaceae (birches and alders) 

 and Salicacese (willows and poplars) are rich in forms pro- 

 duced by parasites of widely different groups. These 

 families are generally regarded as being among the older, 

 and in some respects less specialised, Dicotyledons. 



