552 ALTERATION OF FORM BT CALL-PRODUCING INSECTS. 



the carpels and to the receptacle, and sometimes even to the flower-stalk. All these 

 parts would not have developed as they have done had it not been that the minute 

 quantity of spermatoplasm of a pollen-grain had united with a minute cell in the 

 ovule. 



It will be convenient to consider here the already mentioned similarity between 

 galls and fruits. If the leaf-rudiments in the bud of a Pistacia shrub are not 

 affected by leaf -lice they develop into shining green pinnate foliage-leaves; but 

 if the protoplasm in some of the cells has been altered by the excretions of 

 Pemphigus cornicularius this same rudiment will assume the form of a carpel, 

 and become fashioned into a hollow body deceptively like a pod. The fact that 

 the Pistacia shrub bears plum-fruits and not pods makes it still more remarkable, 

 for the structure arising from the effect of the animal's excretion, when mature, 

 is not like the fruit of the Pistacia, as we should naturally have expected, but like 

 that of a completely different plant species, viz. the Carob (Ceratonia Siliqud). 

 The same is true of the metamorphosis caused by the excretion of a gall-gnat 

 (Lasioptera juniperina) on the uppermost leaves of the Juniper (Juniperus 

 communis) which assume a form very like the fruit of the Arbor Vita? (Thuja), 

 and many other instances might be mentioned in which galls are produced in 

 certain species of plants by animal excretions, looking outwardly very like the pods, 

 capsules, nuts, drupes, and berries of other species. This resemblance to certain 

 fruits is rendered the more pronounced by the development upon the galls of pig- 

 ments, wax-like excretions, and hairy coverings, but of course they contain no seeds 

 in their interior — only the larvae of the animals whose excretions produce the 

 changes of form. The wonderful thing is that the metamorphosis of the growing 

 tissue into a fruit-like body is always of the greatest advantage to the animal 

 which has settled in it, since the tissue serves not only for dwelling and food but 

 also for protection against unfavourable weather and against the attack of foes. 



It is also a fact of great importance that different animals produce different lv 

 shaped galls on the same plant. The Bedeguars produced by Rhodites Rosa', the 

 pea-like galls produced by Rhodites eglanterice, and the clustered protuberances 

 produced by Rhodites spinosissimce may all occur side by side on the same rose- 

 leaf (see figs. 361 *■ 2 - 3> p. 533). On the same elm-leaf Schizoneura Ulmi produces a 

 wrinkled gall, Tetraneura Ulmi a pocket-gall, and Tetraneura alba a covering gall 

 (see figs. 361 4 ' 6 ' 6 ' p. 533). The spherical gall of Nematus gallarum and the bladder- 

 like gall of Nematus vesicator occur close together on the foliage of the Purple 

 Willow (see figs. 361 7 and 361 8 ), and one sees Oak-leaves on which the small 

 spangle-galls of four different gall-wasps, viz. Neuroterus lanuginosus, numis- 

 matis, fwmipennis, and Spathegaster tricolor are all present together (see 

 figs. 364 n - v2 - 13 • u - p. 541). It has been shown that some Oaks, for example, Quercuv 

 pedunculata, may bear as many as 20-30 different forms of gall produced by as 

 manj 7 kinds of gall-wasps. The characteristic shape, colour, and hair-covering of 

 these forms of gall is so constant that we can state with certainty what gall-wasps 

 have given rise to them. These facts force us to the conclusion that the fluids 



