(830) nect the transverse ri"bs v^ith the Gpidermis on top and under- 

 neath. ,0n the other hand, cases are not laoking in which all 

 parts of the ground tissue of the leaf can not increase in 

 the same way. In many fungus galls, the spongy -oarenohyma is 

 superior to the palisade tissue in its capacity for increase. 

 The same difference is evident in many zooceeidia; of a proso- 

 plasraatic char?ioter. Figure 93 illustrafes a (Dipteral) gall 

 of Banister ia, in which the uppermost palisade layer has re- 

 mained inactive, r/hile those lying deeper have divided re- 

 peatedly. 



The epidermis always participates only moderately in the 

 formation of galls. Very often its cells, in contract to 

 those of the mesophyll lying directly beneath them, do not di- 

 TldQ Rt nil; for Instanoe, In the gall of Hormonyia piligera 

 foompare fig. 87) the growth remains Jess than that of the 

 more deeply lyi?ig tissue-layers, so that the epidermis is ul- 

 timately ruptured. In the sac gall of Tetraneura Ulmi the 

 inner epidermis, lining the cavity of the gall, is not able to 

 keep pace with the growth of the otEter epidermis and most 'of 

 the ground tissue layers; Its cells are usually distended, 

 drawn out to "retort" cells with thread-like necks, and finally 

 torn apart (otrapare fig. 97), The cells of the adjacent meso- 

 phyll layers aot in places like this under epidermis. In- 

 stances of this kina, however, are rare. Sac galls and walled 

 galls are usually covered with an uninterrupted epidermis, of 

 which only the sup^rifical growth has been active. 



Epidermal growth in thickness is very much rarer in galls," 

 To be sure isolated cross-diviteions occur in many galls (Urtica, 

 Tilia fig. 96, Jpglans (KustesS, loc. cit.), but only rarely a 

 many-layered epidermis from a single -layered one. The galls of 

 S pathegaster baccarum can be taken as examples, also the flask- 

 iike fly- gal is of the elm (fig. 99) and especially the abtindant 

 willow galls of Nematus gallarum , (j'igure 98). In these, a 

 sheath of tissue oi varying size Is formed from the thin epider- 

 mis; at H, in figure 98, the epidermal tissue seems to be con- 

 strmcted, since no division in the hair cell has taken place at 

 that point, 



I have already called attention to *he fact that no con- 

 clusions as to the participation of the different tissue-forms 

 in the construction of the galls can be drawn from the nature 

 of the host plants and their noirmal histogenetio peculiarities. 

 In the Salix species, the cork is known to be produced from the 

 (231) epidermis. As we have found, the epidermis in many willow 



galls is many layered. To conclude from this that the epidetmis 

 of Salix is especially inclined to cross-division would be ab-' 

 solutely useless. Willow galls (Phytoptocecidia) may be found, 

 in the production of which the epidermal cells remain undivided. 

 On the other has d, in the elm galls above named, we have found 

 that the epidermal cells divide extraordinarily actively, which 

 does not take place under normal conditions and that In Ulmus 

 the cork is produced sub-epidermally - and the like , 



Gfalls have often been compared to tiimors or swellings 

 of the. animal and human body. In fact, in one as in the 

 other, a diseased new formation of tissue is Involved, 

 which shows a moderate similarity in external form in all, 

 Further, in both cases, a similar connection exists between 

 outgrowth and substratum. In galls also, we may speak of 

 "malignancy"; since they often take appreciable amounts of 

 food stuffs from the ground tissue, in this, resembling 

 tumors which, like parasites, use up their substratum and 

 entirely exhaizst it. Besides this very little correspon- 

 dence can be proved. 



