The Vegetative Anatomy of Molinia caerulea. 69 
or next but one or two, to the tooth-bearing cells by the motor 
mechanism (Fig. 8, S), so that when inrolling occurs they are 
quickly closed ; on the under or outer surface they are in rows 
practically opposite to those on the inner surface, and they are 
approximately equal in number. The structure of the stomata is 
shown in Fig. 9, B, C. The guard cells have slightly projecting 
lips, a thick wall by the air passage, and a thin internal wall whose 
distension or collapse opens or closes the aperture. As usual the 
guard cells contain chlorophyll. 
Apart from specially modified elements, both sides of the leaf 
are covered with an epidermis of long but narrow, strongly cutinised 
cells. The mesophyll is composed of chlorophyll bearing cells 
(Fig. 8, a) which are small, polygonal and closely packed in 
transverse section but much more open in longitudinal, especially 
towards the inner surface ; in the middle of the mesophyll the cells 
are slightly larger and more loosely arranged. In the thickest 
part of this thin leaf, to right and left of the midrib, large air 
spaces or lacunae arise by the breakdown of a number of the cells 
of the mesophyll. The wetness of the habitat usually favoured by 
Molinia involves a need of special facilities for aeration, which is 
met in this way, and the consequent weakening of the leaf is 
compensated by the large development of mechanical tissue. The 
small, supporting vascular bundles, previously described, are so 
placed as to give additional strength just where the leaf is weakened 
by the development of these lacunae. 
The leaf-sheath (Fig. 5, C) continues the structure of the leaf- 
blade in a modified form. The stereome of the girdered bundles 
tapers to the inside and expands towards the outer surface. The 
inner epidermis is not cutinised and there are no motor cells. A 
slight amount of chlorophyll tissue develops beneath the outer 
epidermis, but most of the mesophyll is wanting, and instead we 
have a large air space between every pair of bundles, except close 
to the margins where the sheath is too thin. The inner surface of 
the sheath is almost white, an appearance due to two or three layers 
of dead cells lying within the inner epidermis. 
Frequently the bases of the leaf sheaths of the upper series of 
leaves show a prominent swelling. This is a gall produced by the 
larvae of Oligotrophus ventvicoliis, one of the Cecidomyiidae, a midge 
found during the progress of this study to be adundantly associated 
with Molinia, though only once before recorded in England, and 
which has been described by Grimshaw (1) from material supplied by 
the present writer. The galls are present on fully one fourth of the 
