34 Arber .— On the ‘ Squamulae Intravaginales' of the Helobieae . 
passing from below upwards through the basal regions of three successive 
leaves belonging to another apical bud. In Fig. 2 A the first of these leaves 
is just beginning to detach itself from the growing apex ; the earliest sign of 
this detachment is the appearance of the two free overlapping margins of the 
sheathing leaf-base (sk. 1 and sh.\). In Fig. 2 B the free flaps of leaf tissue 
have increased in length, and the first squamulae intravaginales (sq. a) have 
made their appearance. Figs. 2 C and D show further stages. It will be 
seen that the earlier squamules are budded off successively in two series— 
one on either side—from the boundary zone between the shoot apex and 
the free part of the leaf. The points of origin of these two series travel 
farther and farther apart as the sheath detaches itself. Opposite the median 
region of the sheath, however, the squamules become free in the form 
of a continuous zone (marked with a cross in Fig. 2 D) which separates 
almost immediately into constituent individuals (Figs. 2 E and F). In 
Figs. 2 E and F the squamules form a wreath surrounding the growing 
apex, and interrupted only opposite the margins of the ligular sheath 
(tig. j.j) of the first leaf, which is now almost detached from its petiole 
(petf ; this leaf is thus cut slightly nearer its base than such a leaf as that 
marked lam . 2 and tig. s . 2 in Fig. l A. Figs. 2 G and H carry on the history 
to a higher level. In Fig. 2 G the apex (apt) of Fig. 2 F is represented by 
a second leaf ( pet . 2 and tig. ^. 2 ), enclosing the shoot apex, which is surrounded 
by a series of squamules (sq. b). In Fig. 2 H the limb ( lam . 2 ) and ligular 
sheath (lig. s. 2 ) of /. 2 are entirely separate, and another leaf (/. 3 ) with its 
ligular sheath (lig. s.. 3 ) is detaching itself, while a third series of squamulae 
(sq. c ) appear on its inner side. 
A series of sections, similar to those drawn in Figs. 2 A-H, but cut 
from two successive leaves of another species, Potamogeton natans , L., are 
seen in Figs. 3 A-G. The history of the origin of the squamules closely 
recalls that in the species already described. When the development of the 
first of these leaves is followed, however, it is found that there is a slight 
separation between the leaf and the shoot-axis in the median plane, at 
a level at which the flaps of the leaf-sheath are still attached to the flanks of 
the axis (Fig. 3 C). A band of squamular tissue (marked with a cross 
in Fig. 3 C) is thus left temporarily attached to the upper median region of 
the young leaf; but almost at once it becomes free on that side also 
(Fig. 3D). ^In Fig. 3E the ligular sheath (lig. is completely detached 
from the petiole (petf. In this figure, also, we see the first indication of the 
separation of the leaf-sheath of the next leaf (sk. 2 ) from the shoot-apex. 
Figs. 3 F and G show further stages in the development of this second leaf 
and the squamules within it (sq. b ). These squamules remain attached to 
the growing apex—in the median region marked with a cross in Fig. 3 f _ 
even after the leaf has become completely free. 
