| BEN |
| BOTANICAL ELECTRONIC NEWS |
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| ISSN 1188-603X |
| No. 216 March 10, 1999 | aceska@victoria.tc.ca | Victoria, B.C. |
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The Cedar Beach Resort is almost full with new registrations still arriving. If you are interested in attending this gathering of British Columbia and Washington State botanists, and have not registered yet, please give a call to Elizabeth Easton (250-953-3488) or send an e-mail message to Andy MacKinnon (Andy.MacKinnon@gems1.gov.bc.ca). We may have to look for additional accommodation possibilities. - AC
[In BEN # 194 Dr. Ted Mosquin described "explosive pollination" in flowers of bunchberry, Cornus canadensis. Shortly after BEN # 194 was posted, I got a note from Dr. Helen Kennedy about similar explosive pollen distribution in some members of Marantaceae. Helen described it in the following paragraphs. I also learned that, unlike Amaranthaceae, Marantaceae are spelled without "h" - Maranta (arrowroot) was named for Bartolommeo Maranti, Venetian botanist who lived around 1559. - AC]
Marantaceae are found in the lowland tropical forests of Asia, Africa and the Americas. There are 31 genera and about 520 species. The majority of the species, 80%, are in the American tropics while 9% are African and 11% Asian. They are understory herbs found in semi-deciduous to rainforest or cloud forest habitats, ranging from sea-level to 1500 m, rarely to 2000 m. Marantaceae are readily distinguished from all other plant families by 2 distinctive vegetative characters: 1) the sigmoid (S-shaped) lateral veins and the numerous parallel cross veins of the leaf blade and 2) the presence of a region of specialized cells just below the blade, termed the pulvinus, which controls the movement of the leaves. The floral structure is also distinctive and is unique for the family. Given just a single flower, from any species in the world, one can immediately tell it is Marantaceae. The style and cucullate staminode immediately distinguish Marantaceae from all other families.
The style is a curved elastic spring and the stigma is cup-shaped. The style is held under tension by the cucullate staminode which has a trigger. The flowers are borne in a bracteate inflorescence and are usually paired, each bract subtending a single to cymose series of paired flowers, the two flowers of the pair being mirror images of each other. The flower is irregular, with an inferior ovary, the 3 petals fused basally into a corolla tube, a single fertile stamen with a 1-locular anther, (2)3-4 staminodes with an elastic spring style and cup-shaped stigma. There are 2 inner, specialized staminodes, the fleshy, callose staminode and the hooded, cucullate staminode which encloses the terminal portion of the style and stigma and (0)1-2 petaloid outer staminodes. Like the Zingiberaceae and Cannaceae, there is a single fertile stamen and the showy portion of the flower consists of the highly modified sterile staminodes.
The pollination mechanism of the Marantaceae is termed explosive, secondary pollen presentation (Faegri and Van der Pijl 1966). Secondary pollen presentation is another floral character which is shared with the sister group, Cannaceae, but in Cannaceae it is not explosive. The style in Marantaceae is under tension, and springs forward suddenly when released during the pollinator's visit.
The pollen is shed during the evening prior to anthesis while the flower is still in bud, usually between 5:00 and 9:00 pm for species whose flowers open in the morning. The pollen is deposited in a shallow depression on the back of the style just behind the stigma. It is the upward growth of the style which forces the pollen grains from the anther, and onto the stylar depression ("stamp" of Andersson 1981). During the early morning hours the stigma becomes receptive. At anthesis the style is under tension held in place by the cucullate (hooded) staminode which is also under tension pulling in the opposite direction, thus maintaining the style in a static equilibrium. When the pollinator inserts its head into the flower in search of the nectar, it depresses the appendage, or "trigger", on the cucullate staminode thus releasing the style which springs forward bringing the stigma in contact with the pollen (from a previously visited flower) on the pollinator's body and in the same motion depositing its own pollen in the same spot. Lindley (1819, 1826) was one of the first to describe the nature of the floral parts, the pollen transfer and movement of the style. The function of the spring style and the importance of the pollinator displacing the trigger in the pollination mechanism was hypothesized by Hildebrand (1870a). Delpino (1869) actually observed bees ("ape comune") visiting Thalia dealbata Fraser in Florence, Italy. The mechanism, especially in Calathea and related genera is quite precise with the pollen placement in the narrow proboscidial fossa beneath the bee's head. This is one area where the bee is unable to groom or remove the pollen. When the bee's proboscis is retracted after visitation the base of the proboscis covers the pollen, thus protecting it and preventing desiccation.
Each flower has but a single chance to be pollinated, and, once tripped the stylar movement is irreversible. The difference between an unvisited, that is, untripped flower and a tripped one is readily noticeable. Thus, a population can be easily surveyed for the number of pollinated versus unpollinated flowers (whether tripped or non-tripped) with just the use of a hand-lens. Because of the specialized floral morphology, the pollen and the stigma are spatially separated prior to tripping (or pollination) and, as the stylar movement is irreversible after tripping, its position prevents any pollen from subsequently entering the stigma (in nature). This aspect is of great advantage for crossing or selfing individuals in the field or greenhouse as there is thus neither the need to emasculate the flowers prior to pollination nor to bag them afterwards. There are a few exceptions to this which will be discussed later.
Marantaceae floral structure initially evolved in response to bee pollination and the vast majority are still bee pollinated. Differences in the number, shape and sizes of the floral parts, such as length of corolla tube and shape of stigma which are used to distinguish the various genera are often adaptations to different pollinators. The shift to the more precise pollen placement, the protection of the pollen by the cucullate staminode and the distinctive morphological change in the flower following pollination are innovations which may explain the greater species richness or "success" of the Marantaceae versus its sister group, the Cannaceae. A flower differing in aspect following pollination, whether in morphology or color, would contribute to greater foraging efficiency by the bee pollinators and presumably higher seed set for the plant. In several species of Calathea, there is a distinct color change in addition to the morphological change following pollination. The back (abaxial side) of the style changes from an initial white or yellow to dark brown or nearly black giving a clear indication to the pollinator that the flower has already been visited. This was first reported by Hildebrand (1870b) for C. zebrina (Sims) Lindl. and is characteristic of species in the "Calathea capitata group" (Kennedy 1988).
The Neotropical species are adapted primarily to pollination by the long-tongued Euglossine bees (Ducke 1901, Dressler 1968, Kennedy 1974, 1978a), while Old World species are adapted primarily to pollination by the shorter-tongued bees. This adaptation is reflected in the difference of the average tube length for the Neotropical species versus those from the Asian and African continents. The average tube length is 17. 6 mm (N=81) for the Neotropical species while only 4.6 mm (N=81) for the Asian and African species.
In spite of the elaborate pollination mechanism in Marantaceae, there has been a shift to varying degrees of autogamy (selfing) in at least 44 species (ca. 8% of the family). With increased field studies and the introduction of species into Botanical Gardens outside their natural ranges, more occurrences will come to light. Early reports of spontaneous seed set occurring in plants growing in temperate greenhouses or botanical gardens gave a clear indication of the presence of autogamy in the family.
The actual mechanism of autogamy was the same for all cases so far examined, and occurs during the pollen transfer process within the bud. Thus, the pollen grains are already present in the stigma well before anthesis. In most species, the orientation of the stylar tip is such that pollen is prevented from entering the stigma (Fig. 3). A slight change in the orientation of the tip of the style and stigma during this transfer phase can result in a few grains entering the stigmatic cavity while the majority of the pollen grains are deposited, as normally the case, just behind the stigma in the stylar depression (Kennedy 1978b). When the stigma later becomes receptive, early the following morning, the pollen in the stigma can germinate and thus result in fertilization and seed set.
References:
This manual includes a summary of the status of rare and endangered native vascular plant taxa in the province of British Columbia. Over 600 plants are dealt with in this volume with the main body of the book arranged alphabetically by genus/species. It contains the synonyms, common name, habitat requirements, world distribution, provincial distribution map, line drawing and conservation ranking for each rare plant taxon.
The first two volumes, of an eight volume series on the vascular plant flora of BC, have now been printed. These volumes include the Gymnosperms and Dicotyledons (Aceraceae to Asteraceae) [Volume 1] and the Dicotyledons (Balsaminaceae to Cuscutaceae) [Volume 2]. The treatment includes keys to genera and species, habitat/range annotations, technical descriptions and illustrations for each taxon.
This book is a compendium of papers resulting from the Eastern North American Vegetation Survey (ENAVS), a three-year program to survey the flora and vegetation from Quebec to Florida, funded by the Japanese Ministry of Education, Science and Culture. The field work, by a team of Japanese vegetation scientists with assistance from Canadian and American colleagues, included sampling of about 1,300 releves, involving travel of over 20,000 km in the space of a total of fourteen weeks.
The text is divided into five sections (natural environment, comparison of floras, vegetation system and dynamics, ecology, and conservation and rehabilitation) which in turn contain a total of seventeen papers by participants from Japan, the United States and Canada. It is as much a work on phytogeography as it is on vegetation science. An underlying theme in all of the papers is a comparison of Japan and eastern North America from the floristic, syntaxonomic and ecological perspectives.
The book is not only a compendium of existing information on the respective vegetation and floras, but also provides some valuable first-time descriptions for eastern North America. A full 48 phytosociological tables (both releve and constancy) are included for alpine meadows, boreal coniferous forests, deciduous and evergreen broad-leaved forests and mangrove vegetation. The floristic and ecological summarization is accompanied by many black-and-white photographs and a section of beautiful colour plates. All in all, the book is a valuable addition to the knowledge of eastern North American vegetation and will doubtless be an indispensable reference on the topic of eastern North American-east Asian phytogeography far into the future. One could only wish the book was more affordable.