Arber. — On the ‘ Squamulae lntravaginales ’ of the Helobieae. 39 
(sq. d) inside the leaf /. 4 in Fig. 1 C, and in the squamules inside the ligular 
sheaths of two successive leaves from another bud in Figs. 1 E and G. 
In Potamogeton natans the same connexion with the £ axis ’ is seen in the 
case of the squamules marked with a cross inside the inner leaf (skf 
in Fig. 3 F. An exception, however, is illustrated in Fig. 3 c, for here the 
median band of tissue "marked with a cross, which is destined to form 
a squamule, becomes detached from the growing apex before freeing itself 
on the side on which it adheres to the leaf. Despite this one exception, we 
may, I think, claim that the indications in Potamogeton are favourable to the 
view that the squamules, like those of Cymodocea , are derived from the 
surface of the growing apex immediately above the leaf in whose axil they 
appear to be located. 
Leaving the cases which I have myself examined, and turning to the 
records in the literature relating to other Families, we find that the Hydro- 
charitaceae are the group about which we have most information on the 
question of the origin of the squamules. In the case of Stratiotes aloides , L., 
Nolte ( 16 , p. 3, Plate I, Fig. 5) observed that, if the leaves were carefully 
removed, the squamules remained adhering to the axis. Again, although 
Solereder ( 22 ), in describing the squamules of Elodea canadensis , Michx., 
speaks of them as situated ‘an der Blattbasis’, Sanio ( 18 ), nearly half 
a century earlier, had pointed out that in this plant the development indi¬ 
cates that the squamules arise, without connexion with the leaves, from the 
external cell-layers of the stem apex. Furthermore, Bayley Balfour, in 
his beautiful memoir on Halophila (1), explicitly states that, in this genus, 
the squamules f arise from the axis, and have no organic connection with the 
leaves ’. 
In the case of the Aponogetonaceae, the only knowledge we possess of 
the origin of the squamules is Sergueeffs statement ( 21 ) that they arise from 
the stem rather than the leaf. And in the Araceae, which, though not 
members of the Helobieae, show relationship with the Aponogetonaceae, 
Irmisch ( 14 ) found squamules occurring on the axis above the level of 
exsertion of each foliage leaf. 
It seems to me that the indications met with in the literature, as well 
as the observations recorded in the present paper, agree in pointing to the 
conclusion that in the Scheuchzeriaceae, Potamogetonaceae, Aponogetona¬ 
ceae, Hydrocharitaceae, and Araceae—that is to say, in all those Families in 
which we have any definite information about the origin of these struc¬ 
tures—the squamulae intravaginales originate immediately above the leaf in 
whose axil they are found, and arise from the surface of the internode—that 
is to say, from the ‘ leaf-skin ’ belonging to the leaf whose point of exsertion 
lies next above them. It may be objected that, if this interpretation be 
correct, in seedlings of the Helobieae we ought to find squamules belonging 
to the cotyledon situated on the outer surface of the hypocotyl in its basal 
