68 
Rev. T. A. Jefferies. 
transverse section.- In the hollows between the ridges the motor 
cells form groups sharply distinguished by their huge size. Each 
group is composed of three or four large cells reaching half-way 
across the section with a small cell on either side. The outer walls 
are about half the thickness of those of the ordinary epidermal 
cells. The largest cells of the mesophyll adjoin the base of the 
motor mechanism, and frequently, especially near the midrib, appear 
to be losing some of their more solid contents and partaking of the 
nature of the motor cells. On either side of each group is a row 
of cells bearing teeth ( t .), apparently having some definite relation 
to the motor cells: each tooth is an outgrowth of a small 
cell (see Fig. 8, B), which rests on another small “basal” cell 
below and presses against a long cell above, beyond which we have 
another tooth ; these teeth by exposing a surface which is large in 
proportion to the amount of their contents, probably increase the 
sensitiveness of the motor mechanism to changes in the atmosphere. 
Similar rows of teeth are found on the ridges, and others on the 
outer surface opposite the motor cells; in each case there are two 
parallel rows. 
Fio. 9. A. Siliceous plates on epidermal “plate-cells” (P. in Fig. 7). 
B. Stoma in transverse section. C. Stoma in surface view. A x 1300 ; 
B and C x 1000. 
Stomata are freely distributed on both surfaces of the leaf: 
on the upper surface they are distributed in the rows of cells next 
