PROF. J. PERCIVAL ON “ SILVER-LEAF’’ DISEASE. 391 
plum-leaf is given in fig. 1, and the corresponding surface view 
in fig. 2. Here it is seen that the cell-walls of the epidermis are 
Fig. I. Fig. 2. 
| La 
nena cn ( 
SEER Ne 
\ 
Transverse section of the upper epi- Surface view of upper epidermis 
dermis of a normal plum-leaf. of a normal plum-leaf. 
thick and without intercellular spaces. In figs. 8 and 4 are 
illustrated the transverse section and surface view of the epi- 
dermis of a leaf affected with “silver” disease. Large inter- 
cellular spaces filled with air are present beneath the cuticle aloug 
the lines of union of the epidermal cells. The peculiar light 
Fig. 3. 
Surface view of upper epidermis of 
a plum-leaf affected with “silver-leaf” 
Transverse section of upper epidermis 
of a plum-leaf affected with “silver-leaf” 
disease, disease. 
grey colour of the leaves is due to these air-filled spaces, and not 
to any alteration in the chloroplasts; the latter structures are of 
the same size and appearance as those in healthy leaves. 
How these air-channels are produced is not clear at present. 
The appearances presented by the jagged irregular Pieces of cell- 
wall substance which project into the air-spaces (tig. 4) suggest 
