FECUNDITY AND SIZE RELATIONSHIPS IN JACK‐IN‐THE‐PULPIT, ARISAEMA TRIPHYLLUM (ARACEAE)

Arisaema triphyllum is a gender-labile woodland herb in which sex expression is correlated with the abundance of stored resources. Larger plants are female or monoecious, smaller ones are male. Among females larger plants produce more flowers, fruits and seeds, and the rate of successful fruit and seed formation is greater for plants of greater ht and corm diam. Average seed wt is greater in larger plants. Seed number per fruit and average seed wt per fruit taper towards the top of the infructescence. Pollinator limitation and resource supply may both contribute to the regulation of yield; their effects can be interpreted sequentially. SEX EXPRESSION IN Arisaema triphyllum is labile and is correlated with plant size and the abundance of stored reserves in the underground corm (Treiber, 1980; Policansky, 1981; Lovett Doust and Cavers, 1982; Bierzychudek, 1 984a). Larger plants are female (or monoecious) while smaller plants are male or nonflowering. Components of plant fitness such as flower number, and rates of fruit and seed set are closely linked to plant size (for example ht or leaf area). Policansky (1981) pointed out a significant correlation between female fecundity and plant size in A. triphyllum but found no such correlation for males. It has been suggested that a paucity of insect pollinators sets a low limit to the reproductive success of Arisaema triphyllum (Treiber, 1980; Bierzychudek, 1981). Bierzychudek had observed that seed production in A. triphyllum at two populations was not a function of plant size, prompting her pollinator limitation hypothesis (198 1). If we compare the results of different workers, it is apparent that there is considerable variation in seed set from place to place and from year to year (Table 4). Fecundity in A. triphyllum, like sex expression in this species, may also be regulated by the quantity of stored resources. The resources remaining in the corm in fall should be a good predictor of fecundity. Other measures of plant size, such as ht or above-ground biomass, may ' Received for publication 30 April 1985; revision accepted 30 October 1985. We thank Mount Holyoke College for their 1983 faculty research grant to LLD. Jane Leopold carried out preliminary work that was valuable in formulating some of the fruit analyses reported here. David Policansky, Paulette Bierzychudek and Spencer Barrett provided very helpful reviews of an earlier draft. 2 Present address: Amherst College, Department of Biology, Amherst, MA 01002. also correlate closely with yield. In this paper we are concerned with aspects of seed set in jack-in-the-pulpit. We estimate components of fecundity and relate these to plant ht and corm diam, as well as fruit position within the infructescence. MATERIALS AND METHODS-In September 1982, 24 femalejack-in-the-pulpit plants bearing fruits were sampled from the field. Plants were selected randomly from among all females that were fruiting. (Hence, this represents a significant difference from the sampling methodology of Bierzychudek [e.g., 198 1, 1 984a, b] who sampled randomly from among all plants that flowered as females, and would therefore have included some females that set no fruits at all.) The infructescence was removed with the rest of the stem, and the corm and roots were excavated. Plant ht was measured from the corm to the highest part of the plant, and corm diam was measured at its widest part. Individual fruits were removed from each infructescence; seeds were removed from each fruit and their number recorded. Air-dried seeds were weighed. Seeds were tested for germinability and those that did not germinate were tested for viability using tetrazolium chloride. A further 25 infructescences were collected for analysis the following year. The ht of the infructescence and the diam of the peduncle at the base of the infructescence were measured. Individual fruits were categorized as being from the bottom third of the inflorescence, the middle portion, or the top third. Fruits were counted and weighed, and the number and wt ofindividual seeds within fruits from these plants were determined. RESULTSIntact plantsMeans and S.E. for a variety of aspects of growth and yield are