WHAT IS AN "OAK APPLE"? 



695 



examining the contents of each with a lubricant for the working of their ovi- 



magnil>ing glass— I had the good fortune positor, or as a varnish to seal up the 



to disco\er a mass of the transparent, injury — certainly not to poison the tissue, 



pear-shaped eggs of the fly in one of How, then, are we to account for the 



them. The photograph of this bud, with formation of galls ? Probably the irrita- 



the eggs in situ, appears on this page, tion set up by the presence of the eggs, and 



It has not been touched up in any way ; subsequently by the gnawing of the tiny 



thus the reader may see for himself that grubs, is the sole cause. It may be 



the gall-fl}- reall\- lays her eggs right urged that the presence of eggs, or the 



down in the heart of the bud. gnawing of microscopic grubs, seems an 



Now all these buds which contain eggs insufficient reason to account for the 



production of an elaborate gall formation. 



bring forth oak apples in the springtime ; 

 and within these oak apples the 

 young grubs that hatch from the 

 eggs are nourished upon the sap 

 which, strictly speakmg. is pro- 

 vided by the tree for the de- 

 velopment of its leaves. But 

 how is this thing brought about ? 

 In \\hat manner does the gall 

 insect contrive to make the tree 

 provide food and shelter for its 

 offspring ? These questions have 

 puzzled naturalists almost as long | 

 as naturalists have existed to be 

 puzzled. Pliny thought that galls 

 were fungi, and that the flies were 

 bred in them b}^ chance. A later 

 author said that the parent in- 

 sect laid its eggs in the ground, 

 \\'hence they were drawn up by 

 the sap to the lea\-es where the 

 gaUs were found. Still another 

 writer ptit forward the rather 

 obscure theory that " the plant 

 had a vegetable soul, this vege- 

 table soul presiding at the Ongm showing eggs in lower part laid by wingless female of oak apple gall-fly 



of galls, with their eggs, larvse 



MAGNIFIED SECTION THROUGH TERMINAL 

 BUD OF OAK. 



and imagines, while it again gave issue 

 to fruits." A more modern notion, and 

 one which until recently received uni- 

 versal acceptance, was that the parent 

 insect infected the plant tissue during 

 egg-laying — injecting a minute drop of 

 peculiarly poisonous fluid, and that this 

 poison caused the rapid enlargement and 

 subdivision of the vegetable cells affected 

 by it, so as to form the tissue of a gaU. 



Without going into detail, it may be 

 said that this theory — despite its plausi- 

 bihty — does not correctly sohe the 

 problem of gaU formation. Scientific 

 observers have found that although the 

 gaU wasps actually inject a drop of fluid 

 into the wound which they make in the 



But the eggs are invariably laid in contact 

 with what botanists term meristem tissue 

 — that is to say, the particular layer of 

 \-igorous cells whose acti\-e multiplication 

 by fission brings about the phenomenon 

 which we call " the growth of the plant." 

 These cells are known to possess an extra- 

 ordinary sensiti\-eness, and it is not, 

 therefore, unreasonable to suppose that 

 an irritant, however sUght. would divert 

 the energy that in normal circumstances 

 would assist in building up the leaf tissue. 

 Truly a mar\-ellous phase of Nature — 

 that these minute insects should be gifted 

 with the power of diverting to their own 

 uses the cell energy of plants and trees ! 

 Moreover, there is further cause for wonder 



buds, they do this either to provide a in the fact that each of the many kinds of 



