490 JOUKNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Cinnamomum zeylanicum. It occurs chiefly upon the under-surface 

 of the leaves, but occasionally upon the upper leaf-surface or upon J 

 the branches. Infection of the leaf by the mite (Eriophyes Doctersi, ] 

 Nal. in this case) takes place in the bud. The first result of the 

 infection is that the epidermal cells in the immediate neighbour- | 

 hood of the mite grow out into hairs, or become more elongated in 

 shape. A little later a ring of leaf -tissue which surrounds the mite i 

 begins to grow rapidly, so that it builds up a kind of wall around the I 

 mite. The further growth of this wall of tissue produces the gall 

 which encloses the mite within it. The cavity of the gall is at first 

 simple, but subsequently becomes divided up into a number of 

 secondary chambers. The growth of the partitions which produce this 

 subdivision of the gall-cavity is due to the stimulus of the young mites 

 derived from the single one originally enclosed. In older galls the 

 cavity is filled with hairs. The cytological changes in the wall-cells of 

 the gall due to the stimulus of infection are described. 



The second gall dealt with is one on the leaves of Ipomoea Batatas, 

 produced by the attack of an undetermined mite. 



This gall belongs to the class of " pocket galls," that is to say, it 

 is formed not by the growth of a wall of tissue around the mite, but 

 by the leaf surface becoming pushed in where the mite rests, so that 

 it produces a tiny pocket at this spot. The histological and cyto- 

 logical features of the mature and developing gall are fully described. 

 This gall is distinguished from most others caused by mites by the 

 complete absence of hairs in its interior and by its more complete 

 histological differentiation. 



The third gall dealt with is also due to an undetermined mite 

 attacking the leaves of a fern (Nephrolepis biserrata). The develop- 

 ment of this gall is somewhat simiilar to that upon Cinnamomum. 

 In both cases the gall is formed by the growth of a circular wall of 

 leaf-tissue, which grows round and gradually encloses the mite. In 

 the case of Nephrolepis, however, a thickening of the leaf-tissue pre- 

 cedes the development of the ring- wall. No clear cytological altera- 

 tions in the affected cells could be found in this case. 



The latter part of the paper is occupied with a most interesting 

 discussion of some general questions raised by the study of these galls. 

 The classification of galls by other authors and the definition of the 

 word gall " is touched upon. The relative share taken by the plant 

 and the animal in the production of galls is indicated. The authors 

 point out that the character of the gall is in the first place determined 

 by the animal, " the plants only furnish the material from which the 

 animals select, and again build up what they require for the construc- 

 tion of their dwelling." 



The plant tissues possess the potentiality of developing in several 

 directions according to the dictates of external conditions. In the case 

 of gall formation, the stimulus of the infecting organism, so to speak, 

 switches the tissues off the normal course of development and brings 

 to light other, usually latent, qualities of the cells. — R. B. 



