Stomatal Structure in Pandanaceae — Tomlinson 
53 
differently expressed in closely related species, 
it is possible that elaboration is an ecological 
adaptation which has become fixed genetically. 
It is an attractive hypothesis to regard elaborate 
epidermal structures associated in varying de- 
grees with the guard cells as devices whereby 
transpiration is reduced, since this type of mod- 
ification is frequent in plants of dry regions. 
However, in Pandanus an increase in epidermal 
papillosity does not involve a comparable in- 
crease in cuticular thickness. Therefore elaborate 
papillae actually increase surface area and pre- 
sumably also cuticular transpiration. There may 
be overcompensation for this by reduction in 
stomatal transpiration through obstruction of 
the outer stomatal chamber. A more obvious 
antitranspiration device is the plugging of 
many stomata (the "lignified” stomata of Kofler) 
by resinous deposits, although this mechanism 
seems irregular. 
Stomatal elaboration might therefore be inter- 
preted more readily in terms of ecology and phy- 
siology than of taxonomy. Pandanus occupies a 
wide range of habitats (van Steenis, 1956) and 
it would be valuable to examine stomatal struc- 
ture of species occupying contrasted habitats. 
Also the basic physiology of water conduction in 
Pandanus is itself relevant. Pandanus has vessel 
elements with scalariform perforation plates 
bearing many thickening bars in all parts, ele- 
ments of the root being somewhat more spe- 
cialized than those of the leaf and stem. Even if 
the stem is an efficient water conductor, it is 
likely to be subject to water stress as it grows 
older. The leafy crown tends to increase in size 
by branching, but the main axis, being devoid of 
a secondary vascular cambium ( Schoute, 1907), 
has no means of increasing the volume of water 
conducting tissue (Tomlinson, 1964). Reduc- 
tion of water lost through the leaf surface 
would evidently be favourable, regardless of the 
environment. 
No one single factor, therefore, can explain 
stomatal elaboration in Pandanaceae, since it 
may be correlated partly or wholly with several 
unrelated factors, notably phylogeny, ecology, 
and physiology, together with over-all stature 
and growth form. The limited scope of the pres- 
ent study should now be quite clear, as well as 
the enormous scope for future observation. 
MATERIAL EXAMINED 
Pandanus 
(i) Dried specimens from Bishop Museum, 
Honolulu 
P. aimiriikensis Martelli.... St. John 25884 
P. archboldianus Merr. and 
Perry . Barrett 10203 
P. biakensis St. John _..St. John 26142 
P. boninensis Warb Fosberg 31483 
P. capitellatus Merr. and 
Perry Stone 2466 
P. cominsii Hemsl . ...Stone 2549 
P. douglassii Gaud Pearsall 74 
P. dubius Spreng. St. John 25898 
P. erinaceus B. C. Stone. Stone 2578 
P. nemoralis Merr. and Perry. .Stone 2483 
P. nigridens B. C. Stone Stone 2539 
P. odoratissimus Li. var Doty 16791 
P. parkins onii Martelli Stone 2615 
P. pistillatus Martelli Stone 2576 
P. pulposus (Warb.) 
Martelli Stone 1110 
P. whitmeeanus Martelli Stone 2209 
( ii ) Other sources 
Cultivated in Florida: 
P. baptistii Hort. 
P. copelandii Hort. 
P. utilis Bory 
Cultivated at Foster Botanic Garden, Hono- 
lulu, Hawaii: 
P. odoratissimus L.f. var. laevis 
P. patina Martelli 
P. c.f. pygmaeus Thou. 
P. rockii Martelli 
P. sp. {"Halo”) 
Slide collection, Jodrell Laboratory, Royal 
Botanic Gardens, Kew: 
P. aurantiacus Ridl. 
P. furcatus Roxb. 
P. heterocarpus Balf. 
P. microcarpus Balf. 
P. tectorius Sol. 
P. utilis Bory 
Slide collection, Professor V. I. Cheadle, at 
University of California, Davis, California: 
P. utilis Bory Atkins Garden, 
Cienfuegos, Cuba 
P. veitchii Hort Atkins Garden, 
Cienfuegos, Cuba 
