GALLS 397 



face of the affected organ. The third type is where no definite direction 

 of cell division may be found. 



The tissue material used in the formation of galls may be considered 

 from several viewpoints. Thomas asserts that only those tissues are 

 able to form galls which are attacked during development, or in other 

 words permanent tissue cannot form galls and this is certainly true of 

 prosoplasmatically formed galls, but with cataplasms there seem to be 

 exceptions, where callus has been formed from bark parenchyma several 

 years old. Definite experimental proof of the contested points cannot 

 be obtained, because all attempts with experimentally producing cecidia 

 have failed. It is certain, however, that many galls are produced from 

 completely undifferentiated tissue, that is, from the primary meristem 

 of the tips of shoots, or from callus tissue, but not from cells and tissues 

 with lignified walls. It has been proved that all living cells belonging 

 to the epidermis, the ground tissue, or the vascular bundle tissue, can 

 under certain circumstances participate in the formation of galls. The 

 fundamental tissue, or parenchyma, produces the largest mass of the 

 galls, and it should be remarked in passing that the pith, bark and 

 mesophyll cells often proliferate with astonishing luxuriance. If in 

 leaf galls, for example, the infected part of the leaf becomes ten or twelve 

 times the thickness of the normal leaf, it is in nearly all cases the meso- 

 phyll which has been active, for in nearly all galls the tendency to form 

 parenchyma is striking. The epidermis is concerned only occasionally 

 in the formation of galls and the chlorophyll content of gaUs is scanty. 



The comparison of galls with animal tumors has been made but in- 

 advisedly because with the exception of a diseased new formation of 

 tissue being involved and in the absorption of appreciable amounts of 

 foodstuffs from the fundamental tissue galls and tumors have little in 

 common. Galls in contrast to tumors are developed by a typic infection 

 growth. Mixed swellings occur in galls where epidermis, bark, meso- 

 phyll and other tissues unite to form an homogeneous whole while no 

 tumor is known, which consists of characteristic tissue zones of such 

 diversity as those of the galls of the dipterous insects. 



CECIDIAL TISSUE FORMS 



We are next concerned with a study of the different kinds of tissue 

 forms in galls and in their consideration we will treat first the two most 

 important, namely, the protective and nutritive tissues. 



