SCALES OF AQUATIC MONOCOTYLEDONS. 229 
Butomacese: Butomus umbellatus, Limnocharis (Hydrocleis) 
nymphoides. 
Hydrocharidaceæ : Halophila ovata, Stratiotes aloides, Hydro- 
charis Morsus-rane, Vallisneria spiralis. 
I desire to express my indebtedness to Professor I. Bayley 
Balfour, F.R.S., and Mr. A. C. Seward, F.R.S., for material of 
some of the rarer species. 
ZOSTERA. 
In Z. marina the bases of the older leaves form complete 
sheaths for some distance upward from their origin, the combined 
series forming a very much flattened ellipse in section. On one 
side the sheathing base is thin and parenchymatous, on the other 
thick, and contains several vascular strands. The thicker and 
thinner regions of successive sheaths alternate. The axillary 
scales (first described by Bornet (5)) occur at the margins of the 
ellipse, usually two, three, or four at either margin. The scales 
vary much in size, some being quite minute. There are no 
scales along the flat sides of the ellipse, save occasionally in the 
ease of the younger, more central leaves. Adjacent scales 
frequently interlock so that the bluntly rounded edge of one 
scale is embraced by two flange-like ridges of the other. The 
appearance in section thus suggests a ball-and-socket joint 
(Pl. 5. fig. 2). Each seale consists of a pedicel of cells six to 
eight rows in thickness, polygonal in sectional outline and 
taking on a deeper stain than the remaining cells of the scale, 
which are long and narrow (Pl. 5. fig. 4). 
In development the ridge from which the scales arise is multi- 
cellular from the commencement, and in this respect as well as 
in general shape the scale resembles elosely the ligule of Selagi- 
nella. There is, however, no special basal layer such as I have 
described as occurring at the base of the ligule in that genus. 
In Zostera nana the axillary scales are quite similar in appear- 
ance. Usually, however, there is only one scale situated at 
either margin of the ellipse. The vascular strands of the leaf run 
very close to the base of the scale, separated from it, in faet, by 
only 2-3 layers of thin-walled parenehyma, but in no case is 
there any enlargement or expansion of tracheides into a vascular 
cup, so prominent a feature in many species of Selaginella. The 
scales in Z. nana are much shorter than in Z. marina and show a 
narrower pedicel, enlarging into a basal region 8-10 cells thick, 
which tapers gradually into unilamellar margins and apex. 
R2 
