AbstractThe growth, reproductive characteristics and energy allocation to reproductive activities were studied in a Japanese species of jack‐in‐the‐pulpit,Arisaema urashimaHara (Araceae). The stage class structures of this species were based on leaflet numbers and leaf area of individual plants in four different populations in Wakayama Prefecture, Honshu, Japan.Populations ofA. urashimapossess unique male‐biased population structures, with sex ratios (male/female) of 3.14 to 5.00. In this typical sequential hermaphrodite, functionally dioecious species, the switchover from sterile to the male stage orvice versawas found to occur usually in the 11–12 leaflet class, and from a male to female stage orvice versain the 13–14 leaflet class. However, a large proportion of juvenile plants belonging to the 3–4 to 9–10 leaflet classes, which attained 92.3–98.9% in all four populations, may possibly be recruited by a most vigorous cormlet propagation which occurs every season. Seasonal changes in population structure were also examined for five years, from 1981 to 1985, but population structures remained more or less in the same state, although the sex ratios changed from 3.14 to 6.85.Reproductive allocation (RA) was examined at both the flowering and fruiting stages. The RA to total reproductive structures at the flowering stage showed very conspicuous negative correlations to total biomass for both male (r= ‐0.676, P<0.001) and female plants (r= ‐0.861, P<0.001), whereas the RA at the mature fruiting stage was almost independent of individual biomass. The number of propagules produced per plant (PN) was weakly correlated with the individual biomass (r=0.346, P<0.05). The PNper plant exhibited a conspicuously proportional increase in response to the increase in RA (r=0.860, P<0.001). Ovule number per plant, pollen‐ovule ratios, and fecundity were also examined. In spite of a very high number of ovules borne per plant, ranging from 683 to 1902, and sequential hermaphroditism, the maximum fecundity attained was ca. 60%, with production of 560 to 745 seeds per plant. Larger individuals produced larger and more numerous cormlets, a