In March 2006 Janice Kerbel talked to Stephen Harris, Druce Curator of Oxford University Herbaria, as part of her background research for Nick Silver Can't Sleep
Janice Kerbel: I am trying to determine which plants are strictly nocturnal, which have nocturnal properties, and which are nocturnal/diurnal. Is there a known database of plants that are strictly nocturnal? Is there a database of plants with nocturnal properties?
Stephen Harris: No, not as far as I am aware. This material is scattered throughout the literature. The other problem of course is that diurnal properties of plants may change depending on where they are grown. So, a plant may be more or less nocturnal in some parts of the range but not others, particularly if the species is introduced. Nocturnal properties not only include flowering but also leaf movements, e.g. very commonly found in the mimosoid legumes. From the plant’s point of view the important thing is to ensure that the pollen is released and the stigma is receptive. Flower opening is only one part of the reproductive process.
JK: I am searching for plants whose blossoms are open day/night but whose scent is only detectable at night.
SH: Most nocturnal plants will have some scent detectable during the day but this becomes much stronger towards dusk and at night, e.g., species of Nicotiana.
JK: Are there any plants whose nocturnal properties are determined by/parallel with the stages of the moon?
SH: There is no scientific data that I know of on this subject, although there is a lot of folkloric material relating to plants and the phases of the moon.
JK: Do plants have a ‘rest period’ in a 24-hour cycle?
SH: I am not sure what you mean by a ‘rest period’. The other important point of course is that the Circadian rhythm of a plant need not be 24 hours and there is no reason to suppose that all plants have the same ‘internal clock’.
JK: Are there any plants that are believed to be on cycles other than 24-hours?
SH: I do not know of any examples but I don’t see why cycles should not be different between plants, since day length varies between different latitudes.
JK: Are any of these plants specifically male or female?
SH: All of these plants are hermaphrodite…
JK: Are the flowers of Ceiba aesculifolia only open at night?
SH: Yes but the flowers from the pervious night remain on the tree during the day. This pattern is found in other species of Bombacaceae, e.g. Ochroma (Balsa).
JK: Is this for reasons of pollination?
SH: Yes, most are bat pollinated, although insects may visit the spent flowers during the day.
JK: Are there any notes on its ‘blooming’ cycles?
SH: Not that I am aware of.
JK: Does this change with the seasons?
SH: Ceiba tends to flower during the dry season when the tree has lost its leaves.
JK: Are the flowers of Nyctanthes arbor-tristis only open at night?
SH: The records that I can find suggest that this is the case.
JK: What are its pollinators?
SH: Most reports are for long-tongued insects.
JK: From age 16, Philodendron bipinnatifidum blooms once a year. Is this a nocturnal (only) bloomer?
SH: No. The flowers are there all the time. The cycle is based on the opening and closing of the trapping mechanism used to trap the pollinating insects. The inflorescences smell rather unpleasant.
JK: The flowers of Brugmansia arborea ‘swell’ at night – are they also open in the day?
JK: Do they swell for reasons of pollination?
SH: Not sure what is meant by swell in this context.
JK: Is it fragrant day and night?
SH: Yes, although the scent tends to be stronger at night.
JK: Is it true its blooming peaks with full moon?
SH: No real evidence.
JK: Is Cestrum nocturnum open day and night? (Do blossoms ‘flare’ at night?)
SH: Open both day and night but tends to peak at night.
JK: Does its fragrance change between day and night (pleasant at night/unpleasant in day)?
SH: It is a case of increasing at night, as with many night-flowering species. The ‘pleasant’ scent of the blossom likely masks the ‘unpleasant’ scent of the leaf.
JK: Are all flowers of the yucca family nycitropic?
SH: No, as far as I am aware.
JK: Do the flowers of Yucca baccata/filamentosa/olauca/augustifolia actually move with the moon, or do they simply face the moon in the evening?
SH: There is no convincing evidence of synchronic movement.
JK: Are the flowers of the yucca open day and night? Do they smell during day and night?
SH: They’re mainly open at night and producing nectar for bats. Again, scent is stronger during the night.
JK: Are there other nycitropic plants?
SH: Many, especially mimosoid legumes with their leaf movements.
JK: Is Hesperaloe nycitropic?
SH: No that I know of.
JK: What happens on nights when there is no moon?
SH: Yucca species flower with or without the moon.
JK: Are the flowers of Dictamnus albus open day and night with the gaseous smell at night only?
SH: The flowers are open all the time. The scent is due to volatiles in the leaves and these are present all of the time. They may be more detectable at night due to less air movement.
JK: Do all day lilies (Hemerocalis) open in the late afternoon then fade by morning?
SH: Most species do, but the timing will depend on the species and latitude from which they come.
JK: Are some species more nocturnal than others?
SH: This is not a genus that I know very well but there is a book by A.B. Stout called Daylilies published in 1934 that may provide some information.
JK: Is Nicotiana sylvestris diurnal/nocturnal?
SH: As for any significant nocturnal changes, the scent tends to get much stronger at night.
JK: The Oenethera blooms after nightfall – but when does it close? Is closing time determined by the rise of the sun/fall of the moon?
SH: The flowers tend to open but not to close. I do not think there is enough data to implicate either the sun or the moon – it may be associated with environmental conditions created by such events.
JK: Are some oenethera more nocturnal than others?
SH: Yes, this is a large complex genus.
JK: Are there any plants other than Monotropa uviflora that have no chlorophyll?
SH: Yes, e.g., in North America plants such as Beechdrops (Epifagus virginiana) and, in Europe, broomrape (Orobanche spp.), Cistanche and Neotina. Other species that might be of interest include Mirabilis jalapa and Selinicereus grandiflorus.