Stomatal Structure in Pandanaceae — Tomlinson 
41 
rectangular, slightly longitudinally extended, and 
arranged in longitudinal files. In the abaxial, but 
not adaxial, epidermis distinct nonstomate costal 
regions below the veins are differentiated from 
intercostal regions, between the veins, to which 
stomata are restricted. There is no coincidence 
between epidermal and hypodermal files. Hypo- 
dermal cells contrast with epidermal in that their 
outermost two layers consist of flattened cells 
extended transversely and not longitudinally to 
the long axis of the leaf. Since epidermal cells 
are thus much narrower than hypodermal cells in 
the transverse direction, three or four files of 
epidermal cells occupy the space of one file of 
outer hypodermal cells ( Fig. 2 ) . Anticlinal walls 
in outer hypodermal layers do not coincide since 
their cells are clearly derived from different 
initial layers. The outermost hypodermal layers 
often become sclerotic; the inner cells in con- 
trast are larger, more cubical in shape, and 
usually remain thin-walled. Hypodermal layers 
are continuous except for well-developed sub- 
stomatal chambers by which the stomata com- 
municate with the internal leaf atmosphere. 
The mesophyll includes equivalent, parallel, 
longitudinal veins, separated by assimilating 
layers which surround pseudolacunae. Each vein 
is supported by a well-developed parenchyma- 
tous and fibrous buttress, continuous with the 
hypodermis of each surface. Vascular tissues are 
quite complex but are not considered further in 
this account. The mesophyll between the veins is 
represented by large colourless cells which tend 
to be arranged in separate transverse plates. 
These cells collapse somewhat in mature leaves, 
so that in thin transverse sections there is an 
apparent lacuna between each adjacent pair of 
veins. These pseudolacunae are lined by small 
chlorenchyma cells, the adaxial layers often con- 
spicuously palisade-like. The lacunae are also 
traversed by compact transverse diaphragms, 
mostly two cells thick. At frequent intervals 
these diaphragms include transverse vascular 
commissures which anastomose with the longi- 
tudinal veins. Strands of fibres, independent of 
the fibrous extensions of the bundle sheath, oc- 
cur in a number of species, usually in the super- 
ficial layers. 
In the subsequent description an attempt has 
been made to employ a consistent terminology 
based on that used by Florin (1931). However, 
Florin’s terminology refers to gymnosperms. 
Stomata in Pandanaceae are tetracytic in the 
sense used by Metcalfe (1961). The cells next 
to the guard cells are subsidiary cells, of which 
there are four: two lateral subsidiary cells which 
are very different from normal epidermal cells, 
and two terminal (or polar) subsidiary cells 
which are less different from normal epidermal 
cells. The cells which immediately surround the 
subsidiary cells may be termed neighbouring 
cells, but the account of stomatal development 
below shows that neighbouring cells have dif- 
ferent origins. They correspond approximately 
to the Kranzzellen of Florin, who uses the term 
"neighbouring cell” ( Nachbarzell ) collectively 
to signify any cell which surrounds the stomatal 
apparatus and differs from a normal epidermal 
cell. However, it is difficult to apply Florin’s 
terminology strictly to Pandanaceae. Neighbour- 
ing cells in Pandanaceae vary in size and number 
and may or may not differ in varying degrees 
from normal epidermal cells. 
STRUCTURE OF ABAXIAL STOMATA 
Adaxial stomata are often different from 
abaxial stomata and are described briefly later. 
Stomatal development has been described in 
Pandanus graminifolius by Pfitzer (1870) with 
an emphasis on the position of stomatal mother 
cells in relation to the hypodermal layers. Pfitz- 
er’s observations have been confirmed by my 
own study of stomatal development in four Pan- 
danus species, using the method described by 
Stebbins and Khush (1961). Stomata always 
originate from epidermal cell files above the 
longitudinal anticlinal walls of the outermost 
hypodermal layer, but directly above the next 
innermost hypodermal layer (Fig. 2). Transverse 
divisions continue longest in future stomatal 
files, so their constitutent cells are somewhat 
shorter, as well as wider, than cells in non- 
stomatal files. First evidence of stomatal differ- 
entiation occurs not in the epidermis itself but 
in the outermost hypodermal layer, by the de- 
velopment of substomatal chambers below fu- 
ture guard-cell mother cells (guard mother cells). 
Pfitzer emphasises this peculiarity and suggests 
that development of this internal cavity de- 
termines the presence of a stoma. The two hy- 
podermal cells between which the chamber 
