32 Arber.—On the ‘ Squamulcie Introvaginales ’ of the Helobieae. 
elements. There is great variety in shape, and the appearance may 
be modified by a fringe of marginal hairs. But, despite this astonishing 
range of variation as regards number, form, and structure, the squamules 
all agree in the lack of vascular tissue, and in being apparently of a trichome 
nature. 
Although we possess such a large amount of information about these 
squamules, the literature often reveals a certain vagueness on the practical 
question of their exact point of origin, and on the theoretical question 
of their morphological interpretation. They have generally been spoken of 
as ‘ intravaginal * or ‘ intra-axillary ’, or as occurring ‘ at the leaf-base *, 
except in certain cases in which they have been observed to take their 
origin from the axis. Caspary (5), in 1858 , called them stipulae intra- 
foliaceae , while Prillieux (17), in 1864 , described them as very small stipules, 
and in more recent times Buchenau (4) has suggested that in the Alismata- 
ceae they should be regarded as ‘ Ligulargebilde \ Bornet (2) treated them 
as appendages of the leaf above them on the axis, while Irmisch and appa¬ 
rently all the other writers who have considered the squamules hold them 
to belong to the leaf below them, in regard to which they occupy a more 
or less axillary position. That some obscurity and uncertainty should 
exist on these points is not, however, surprising, since the greater part 
of the work on the subject predates the general employment of the micro¬ 
tome in botany; and, without serial sections, one could scarcely hope to 
arrive at a complete understanding of the relations between these minute and 
delicate squamules and the rest of the shoot. The object of the present 
paper is, with the help of the microtome, to trace the history of the develop¬ 
ment of the squamulae intrav agin ales in a small number of cases from 
among the Helobieae, in order to see if it be possible in this way to obtain 
a firmer basis for the interpretation of these structures. 
I am indebted to Professor Ostenfeld, of Copenhagen, for- material 
of Cymodocea isoetifolia , Asch. 
(ii) Description of Observations. 
Potamogetonaceae — Potamogeton. 
Fig. 1 A gives a general idea of the appearance of the squamules among 
the leaves, as they are seen in a transverse section of the inner part of 
an apical bud belonging to a broad-leaved, submerged species of Potamogeton 
(unidentified). In the case of the outermost leaf represented, only the ligu- 
lar sheath (tig. s.j) is included ; a large number of squamules (sq. a) are seen 
between it and the next leaf. One of these squamules—that indicated by an 
arrow in Fig. 1 a —is shown on a larger scale in Fig. 1 B. The leaf inside 
this set of squamules is cut through the base of its limb ( lam. % ) and ligular 
sheath (tig. s. 2 ), on the inner side of which another set of squamules (sq. b) 
