306 Ward . — On Relations between Host and Parasite 
that the stomata are more numerous on the upper surface 
than on the lower, the increased numbers being due to closer 
packing of the stomata in each longitudinal series, and occa- 
sionally to an increase in the number of series above. The 
stomata are large, and always placed with the long axis 
coinciding with that of the longitudinal series. As a rule 
the stomata are somewhat larger on the lower than on the 
upper surface, and account has to be taken of some variation 
in the extremes of size: the majority, however, are of an 
average size between these extremes. 
Hairs are never totally absent, though they vary enormously 
in abundance, and the rule is almost without exception that 
they are more numerous and often much more numerous 
below than above, thus reversing the order of events found 
in the case of the stomata. 
As regards the length of the hairs, considerable variation 
is also evident, though sufficient constancy as regards average 
length is found in each species to make it worth while noting 
the character. The hairs are always simple and usually stiff 
and tapering, and arise from the crests of the ridges on the 
upper surface of the leaf, but from the flanks or from the 
intervals (grooves) between the ridges on the lower surface. 
On the latter, moreover, there is no recognizable order in 
their arrangement, and beyond a sort of general rule that 
a stoma and a hair do not usually occur close together I have 
been unable to observe any very definite relation between 
these organs. At the same time it appears probable that 
in their inception there is some more or less definite order 
of succession, but since I have paid very little attention to 
the youngest stages, the matter must remain undecided. 
All the Bromes examined have motor-cells in the grooves 
between the ridges on the upper surface ; there are none on 
the lower. These motor-cells are merely enlarged and turges- 
cent epidermal cells, the contraction and expansion of which 
draws together or drives apart the ridges of the upper surface, 
thus bringing about corresponding infolding or expansion of 
the leaf. The mechanism is easily seen in the behaviour of 
