230 PROF. R. J. HARVEY GIBSON ON THE AXILLARY 
PoTAMOGETON. 
The axillary scales in Potamogeton perfoliatus differ markedly 
from those of Zostera, being in the form of plates of parenchy- 
matous cells, usually two cells in thickness throughout the 
greater part of their extent, and extending laterally in the leaf- 
axil for a distance of about two-thirds of the insertion of the 
leaf. Developmentally the scale arises from a single row of cells 
(Pl. 6. fig. 7), which very soon divides into two layers (fig. 8). 
Distally these mitral cells divide frequently, until at a short 
distance from the axil the scale becomes 6-8 cells thick. The 
remainder—by far the greater part—of the scale is two cells in 
thickness, becoming at the apex unilamellar. The vascular bundle 
entering the leaf, although it does not show any tracheidal cup, 
thickens slightly by developing in that situation one or two short 
accessory tracheides. 
In Potamogeton crispus the axillary scales are much smaller, 
but are identical in general character with those of P. perfoliatus. 
Their bases also are not so much swollen. 
The general features of the scales in this genus were first 
described by Irmisch in 1858 (6). 
Ruppia and APONOGETON. 
In an unnamed species of Ruppia from Rodriguez, collected 
by Prof. Bayley Balfour, and in Aponogeton distachyon the scales 
are solitary in the axils of each leaf and arise from a double row 
of initial eells. Older scales in Aponogeton show a certain 
amount of cutinization of the basal cells of the scale, a feature 
to which I will refer under Hydrocharis. 
Dutailly (7) records the occurrence of scales (in Aponogeton) in 
the axils of consecutive leaves, and speaks of them as exhibiting 
intercalary growth. This mode of growth is common to all cases 
which I have examined. 
TRIGLOCHIN. 
In Triglochin maritimum the axillary scales are large and 
broad, and arise from the base of the sheath of the leaf rather 
than from the axil. In the allied species Triglochin palustre 
the scales are more of the type seen in Potamogeton and the 
basal cells of older seales show marked cutinization. 
