214 DISEASE IN PLANTS. 



that the parts in contact with the animal are 

 arrested in their growth, while those further awa)- 

 grow more rapidly^^.^^g. where Mites, etc., cause 

 puckers and leaf-rolling. In true galls the hyper- 

 trophy may consist merely in the enlargement of 

 cells already present, and no new cell-divisions and, 

 still less, changes in the nature of the tissues 

 result e.g. some pocket galls on Viburnum, 

 Pyrus, etc., and the hairy outgrowths of the 

 epidermis known as Erineuvi. In other cases 

 there is not only hypertrophy of existing cells, 

 but new cell-divisions are instituted : these cell- 

 divisions may be confined to the direction per- 

 pendicular to the epidermis, and the tissues grow- 

 only in the direction of the surface, producing 

 puckerings e.g. the Aphis galls on Rihes, Phy- 

 toptus galls of Salvia, leaf galls on Tilia, Acer, 

 Alnus, etc., and the curious galls on Plums due to 

 Cecidomyia Pruni, and which must not be con- 

 founded with the " pocket plums " and similar 

 galls due to Exoasci, 



In a third series of cases, cell-divisions occur 

 parallel to the surface of the leaf, and galls are 

 formed which grow in thickness, and develop the 

 most extraordinary and complicated new tissues 

 proteid-cells surrounding the egg or larva 

 deposited inside, followed by a protective layer 

 of sclerenchyma encasing this food layer, and 

 around this again softer tissues which may 

 assume the structures and functions of respiratory 

 tissues, water-storing tissues, starch reservoirs, 

 assimilatory, or protective tissues of various kinds. 



