HYMENOPTERA OAK GALLS. 133 



The two important factors at work in connection with gall-formation 

 are the activity of the vegetable sap on the one hand and the influence 

 of the animal agency on the other. Botanically considered, gall- 

 formation cannot take place unless in direct contact with vital cells 

 that is, those cells specially set apart for growth and development. 

 In the woody portion of the tree, from the root to the extremity of 

 every branch, there is a special zone of formative cells known as the 

 cambium ring which plays the part of growth in the theatre of life. 

 A section of leaf shows practically two layers of cells, a layer of 

 closely packed cells on the upper side, known as palisade cells in 

 consecmence of their resemblance to a paling fence ; and a sort of 

 double layer on the under side, arranged in opposite direction to the 

 palisade cells. The under cells of the leaf are considered to be those 

 most actively associated with growth and assimilation. When eggs 

 are deposited in a normal winter bud, before there is any differen- 

 tiation of structure between the upper and under side of the leaf, we 

 often find the gall structure on both sides of the leaf, but the pre- 

 ponderance of structure is always on the under side. This is notice- 

 able in the case of Sjmthegaster baccarum. In order to fully under- 

 stand the formation of gall structure on different parts of the tree, as 

 associated with special cells, it is obvious that recourse must be had 

 to section-cutting with the microtome, as well as to minute micro- 

 scopical investigation. 1 



As gall-formation cannot take place unless in contact with special 

 cells, it follows, therefore, that the insect must deposit the egg with 

 the greatest exactitude, otherwise abortive galls would be the result. 

 In the artichoke gall, for example, a species formed in buds, we 

 often find many abortive galls. This may be the result of an egg- 

 being deposited in a bud which had been previously pricked by 

 another gall-forming insect, and the vital cells of the plant thus be- 

 coming injured ; or the premature death of the larval creature within 

 the gall may have arrested the development of the gall. Considering, 

 from the entomological side, the effect of the insect on the vegetable 

 structure, it has been proved that the wound resulting from egg-laying 

 is at once healed up, and that there is, according to the species, a 

 considerable lapse of time from egg-deposition to hatching. During 

 this period the egg derives oxj^gen from the air, through the medium 



1 For a full account of gall structure see ' Pathologische Pflanzenanatomie,' 

 von Dr Ernest Kiister. 



