II. Histological 199 



^j^^ Since we now understand clearly the external form of the 

 different species of galls, so far as of interest here, we win 

 now tu/n to the results of raioroscopic research; we will first 

 look more closely into the life history of the galls and then 

 investigate the dififerent kinds of tissues in mature ones. 



If we study the ahnorraal cell->divigions which usher in the 

 formation of the galls, we can distinguish three tjipes, accord- 

 ing to their direction; a regular orientation of the cross- 

 walls can not be recognized in young galls, or the cells divide 

 predominantly perpendicular to the upper surface of the organ 

 involved, or they divide chiefly or exclusively parallel to the 

 surface of the organ. 



The galls of 0;mips t erminalis which consist of irregular 

 callus tissue (compare fig, 84) furnish an example for the 

 first case in which all regular orientation of the cross-walls 

 is absent in the young galls. Ho definite direction of cell- 

 division may be found in the young walling^in rolls. 



We find division predominently perpendicular to the upper 

 surface of the organ which bears the gall in the galls produced 

 (225) by growth parallel to this surface;- in sac galls. In investi- 

 gating early stages of development, cell rows are often found 

 running tangent ially which have originated from one cell by 

 anticlinal division. Figure 92 illustrates this case by a ovoss- 

 seotion through the gall of Pemphigus marsupialis (on poplar 

 leaves). In most so© galls , numerous pe ri clinical divisions 

 are added to anticllnical ones; However, the galls of Oecid- 

 omyia bursaria (on Glechoma) for example, seem to be produced 

 practically exclusively by anticllnial walls. At least in the 

 examples which I have investigated, I could find bnly here and 

 there an isolated periclinial wall, 



Most medullary galls may be traced bgck to divisions par- 

 allel to the upper surface of the or^n which bears the gall. 

 Instances are not rare here, in which extensive galls are pro- 

 duced exclusively by cell division in one direction. Even in 

 mature, ripe specimens at times thfe regular arrangement of the 

 cells in rows leaves no doubt on this subject. Figure 93 shows 

 part of a cross-section through an undetermined (Biptera?) 

 gall of Banister ia^-. Leaving the epidf?rmis and upper palisade 

 layer of the mesophyll (P) ftut of the question, all the cells 

 of the leaf tissue have divided extraordinarily actively and 

 have produced long, strikingly regulai- cell rows. Cell divi- 

 sion does not always take plaod i^qually intensively in all 

 parts of the gall, rather, It is strongest in the middle and 

 weakest at the edges. In this way, many flat elliptical galls 

 are produced on leaves f Ceoidomyia tiliacea on IPilia, Hormonyl a 

 Caprea on Saljx Caprea etc.). In cross-section, the cells are 

 fouM to be arranged in regular rows, which, running in straight 

 lines at the center, are rolled up at theedges, turning their 

 concavity toward the periphery (compare, fig. 94); the cell rows 

 and the outlines of the gall may be considered a system of 

 orthogenic trajectories. 



(286) The distinction between galls producedo predominantly by 



anticlimal division and those others produced entirely by perl- 

 clinal division still has an especial significance, in so far 

 that the histology of galls of the first kind always remains 

 simple, while in the others, besides simple structural condi- 

 tions, extraordinarily complicated ones also may be found, 



From the herbarixira in Munich, 



