A REVIEW OF ALOCASIA (ARACEAE: COLOCASIEAE) FOR THAILAND
of inflorescences follows upon the first, and a third and fourth and so on each arising from the axil of the prophyll subtending the previous pair. Eventually the flowering episode ceases and the last relay axis bears a foliage leaf which emerges from the centre of a larger or smaller sympodial cluster of inflorescence pairs - up to about 20 in robust species such as A. sarawakensis M.Hotta (Borneo). After rapid resumption of vegetative growth the stem may bear a ring of physiognomically lateral infructescences below the leaf crown of the new vegetative module, e.g., in A. robusta M.Hotta (Borneo, Anambas Islands).
The spathe is divided into a convolute thicker lower portion – the ‘lower spathe’ – which houses the female zone of the spadix and which persists into fruiting and a thinner, ephemeral open limb (this part is also convolute and persistent in some East Malesian species of the Xenophya Group). The two portions of the spathe are differentiated by a constriction so that the lower part is globose to ovoid. The spadix, as is the general case in Araceae, is protogynous and at the time of stigma receptivity the spathe constriction loosens, providing pollinators access to the pistils, simultaneously the inflorescence may emit a detectable scent – highly fragrant to an odour of decay. At this time the spathe limb is generally erect. At the end of female anthesis, the spathe constriction closes and grips the spadix and scent production ceases. There is a sterile zone between the male and female zones of the spadix and typically the spathe constriction is level with this so that after female anthesis the female zone is isolated from the male zone. Male anthesis then occurs. The pollen is mealy and drops to collect between the lip of the limb and the spadix or in a trough-like annular fold that has developed at the base of the limb.
The female zone of the spadix consists of naked pistils. The sterile zone, or interstice, is partly or entirely covered with truncate neuter organs (synandrodia), which often but not always appear to be of two types. The lower whorl(s) (with respect to the spadix) may be composed of smaller, often longer structures than the upper ones and they commonly react differently (remaining white) in alcohol to the larger upper ones which closely resemble the male flowers except for the absence of pollen thecae. This differentiation of the neuter organs is much clearer in Alocasia odora, where the lowermost neuter organs are not in connate groups, but instead partially encircle the uppermost pistils clearly in the positions of staminodes. The next whorl (with respect to the spadix) of neuter organs consists of united ‘staminodes’ with a central hole, seemingly where the pistil would be. There is then one or more similar whorls followed by an abrupt transition to structures resembling sterilised synandria (termed synandrodes). The rhombohexagonal synandria – fertile male flowers – are generally 4–6-merous and consist of connate truncate stamens. The body of the male flower is here termed the synconnective and the vertical pollen thecae are attached throughout their length to its flanks. Typically the thecae reach the top of the synandrium and open through apical pores. However, in some species the synconnective is expanded over the top of the thecae, which release pollen from apical slits into the spaces between the synandria.
The upper part of the spadix forms a well-developed sterile appendix, which is at least sometimes thermogenic (as the male zone may be). The appendix surface is occasionally smooth, but is more usually sinuously, longitudinally and finely channelled -apparently formed of irregular elongate compressed synandrodia.