— io5 



can be observed near the centre of each a minute hole through the 



the epidermis of the leaf, 

 enlarged at c. in Fig. i.) 



% 



(Two of these spots are represented 

 This hole leads into a cavity within 

 the substance of the leaf ; and in this cavity reside the mites that 

 cause the disease. 



Minute Structure of the Leaf Gall. — The diseased parts of the 

 leaf within which the mites live may be termed galls, as are the 

 various abnormal vegetable growths produced by true insects.* 



These galls, or blisters, as 

 they are sometimes termed by 

 gardeners, are swellings of the 

 leaf, within which there is a 

 cavity affording a residence for 

 the mites. Fig. 2 represents 

 a section of a leaf through one 

 of these galls. Here the leaf 

 is seen to be greatly thickened 

 at the diseased part. On the 

 lower side there is an opening through which the mite that 

 started the gall entered, and from which young mites developed 

 in the gall can escape, in order to start new galls. In addition to 

 the swelling of both surfaces of the leaf its internal structure is 

 seen to be modified. In some parts there is a great multiplication 

 of the cells, and in others a large part of the cells have been des- 

 troyed. Two eggs of mites are represented in this gall (e). 



As the season advances and the galls become dry and brownish 

 or black, the thickening of the leaf becomes less marked. In 



Fig. 2. — Section of leaf . g. Gall ; 71.11. 

 normal structure of leaf ; o. opening 

 ef gall ; e. eggs, after Sorauer. 



Fig. 3. — Section of leaf showing structure of gall in autumn. 

 g. Gall; n. uninjured part of leaf ; o. opening of gall. 



fact in some cases there is a shrinkage of the parts affected. 

 Fig. 3 represents a section through a leaf collected and studied 

 in October. 



*Many writers apply either the term cccidia, or acarocecidia to galls pro- 

 duced by mites ; bm we do not see the necessity for this multiplication of 

 technical terms. The same writers refer to diseases of plants caused by 

 mites as acariasis, or if produced by gall mites (Family Phytoptidcc) the 

 disease is termed phytoptosis. 



