CUTICULAR STRUCTURE OF FSILOPHYTON. 379 
Scattered among the epidermal cells are stomata, whose exact structure Is 
rather difficult to make out (see Pl. 37. figs. 2, 3, 6 & text-fig. 2). They are 
not very numerous, for there are about 4-9 per square millimetre. They 
are all orientated in the same direction, with the long axis vertical. They 
usually appear to have an elongated pore, the walls bounding which are 
thickened and cuticularised. The outer walls of the cells flanking the pores 
are also thickened and frequently have a crescentic form. In some cases 
the pore seems to be closed, and there is only a single dark longitudinal 
line between the two outer crescentic thickenings (text-fig. 2). It seems 
Fia. 3. 
P. princeps, Spine-bise from which the spine has been torn away. Slide V. 14918 e, 
Brit. Mus., Geol. Dept. 
probable that the guard-cells were cuticularised along their inner and outer 
walls, the thinner portions of the walls above and below the pore having 
disappeared. It is just possible, however, that these cells with thickened 
walls are subsidiary cells and that the actual guard-cells were on a slightly 
lower plane and have not been preserved, unless indeed the inner thickenings 
alone belong to the guard-cells. I have so far been unable to obtain any 
sections which would throw light on the structure of the guard-cells. As 
regards their dimensions, the average length of the stomatal apparatus is 
07 mm. and the average width ‘03 mm. 
LINN, JOURN.—BOTANY, VOL. XLVI. 2F 
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