Wood-poppy (Stylophorum diphyllum) COSEWIC assessment and status report: chapter 6
Biology
Life cycle and reproduction
Stylophorum diphyllum is a polycarpic perennial. Each plant produces one to several flowering stems. The maximum number of stems counted on a single individual is 19, the average 8.3. The number of flowering stems is strongly correlated with the size of the plant measured by the spread of the canopy. Ex situ plants can flower in their first year, but in the wild no flowering was detected on plants with a canopy spread of <20 cm (Bowles, 1997). Plants grown in ex situ plantings or tagged in the wild will flower annually, but begin to decline and show dieback in the centre of the root crown after 4-5 years. Flowering is indeterminate, but peak flowering occurs over a period of about 10 days in mid-late May. Usually each stem produces 1-4 flowers, but additional flowers and even flowering stems can be produced after the peak season and into the fall. This is much more common in ex situ populations than in the wild. The average number of full seeds produced per capsule was 39, with a range of 0 to 99. Consequently, an average-sized plant can produce over 1000 seeds per year.
Stylophorum diphyllum follows a phenological pattern exhibited by many “summer green” deciduous forest herbs. Vegetative growth is apparent in mid- to late April in southern Ontario, and by late May or early June flowering is completed. The leaves remain photosynthetically active until early fall, by which time the fruits have disintegrated and the seeds dispersed.
Flowering dates of herbarium specimens housed at University of Michigan Herbarium (MICH) show the expected relationship with latitude. The earliest date on a flowering specimen was April 11 from Cheatam County, Tennessee (Latitude approximately 36°10'N) and the latest date was June 18 on a specimen from Antrim County, Michigan (Latitude approximately 45° N). Lindsey and Newman (1956) reported a 30-day range in first flower date with a 30-day growth period, a temperature threshold of 10°C and a sum of about 1150 degree hours required for flowering. In Canada the first flowering date is usually in the first or second week of May (Bowles, unpublished data).
The seeds of Stylophorum diphyllum have a fleshy, white, oily endosperm and rudimentary embryo. In germination studies, Baskin and Baskin (1984) found that most seeds germinated in the spring following production. The pattern of dormancy was quite complex involving both morphological and physiological mechanisms. Seeds with undeveloped embryos are classed as morphologically dormant while those with physiological inhibition are classed as physiologically dormant. A combined morpho-physiological dormancy appears to be present in Stylophorum diphyllum. Temperatures of 5ºC were required for embryo growth prior to germination.
In 1998, seeds of Canadian populations that were air dried and kept in a refrigerator at 3°C from June to october and then overwintered outside in flats in a cold frame had a germination rate of over 80% the following year. Subsequent attempts to germinate seeds using a similar method have not been successful, and no germination has been detected in seeds planted in marked locations at the Stylophorum sites (Bowles, unpublished data).
Predation and disease
A limited amount of browse has been observed on wild plants. Usually capsules are removed, either by white-tailed deer (odocoileus virginianus)or by woodchucks (Marmota monax). The acrid yellow sap probably deters many potential predators. Very little browsing by invertebrates (such as insects and molluscs) has been observed in wild plants. Most mortality in adult plants appears to follow softening and rotting from the centre of the root crown. It is not known whether this is a disease or simply decay that follows senescence in older plants. Seeds are also eaten by mice.
Dispersal/migration
The seeds of Stylophorum diphyllum are about 2 mm long and 1.5 mm wide with a carunculate aril (Gunn and Seldin, 1976; Gunn, 1980) believed to be an adaptation to dispersal by ants (Nordhagen, 1959). The seeds have a fleshy, white, oily endosperm (Figure 4). In ant-dispersed species the ants are attracted to the fatty acids in the elaiosome and typically carry the seeds back to the colony where the oil-rich organ is removed and the seed discarded. Seeds placed on a concrete path in a garden were removed by ants in less than 1 minute (Bowles, personal observation). At the London site 79% of 600 seeds placed out on small trays had been removed within 24 hours. When half the seeds were covered with wire mesh cage to prevent predation by rodents, 25-33% of the protected seeds were removed at the London site, but at Fanshawe no protected seeds were removed. This suggests that ants were not present at Fanshawe, or were not collecting the seeds, which could explain the very low recruitment rates at this site during that period. Unprotected seeds at Fanshawe were apparently mostly eaten by mice. Mice droppings containing the seed coats were found on the trays used for the experiment (Bowles 2000).
Adaptability
Although the flowers of Stylophorum diphyllum are large, showy, and are visited by insects, Elrod (1904) found that seed set was normal in flowers isolated from insects.
According to Baskin and Baskin (1984), Stylophorum diphyllum does not reproduce vegetatively, although spread of individual plants does occur by rhizome growth. However, rhizomes are very short and stocky, and spread of individual plants is very limited.
Baskin and Baskin (1984) suggest that germination, like that in Stylophorum diphyllum, which requires a cold stratification period, is adaptive for deciduous forest herbs because it allows seedlings to avoid the extreme winter temperatures, but to take advantage of the vernal light phase before canopy closure.
Figure 4. Photograph of Stylophorum diphyllum seed, showing the fatty elaiosome of translucent cells.
