植物研究雑誌 J. J. Jpn. Bo t. 79: 79: 326-333(2004) Possible Possible Role of the Nectar-Guide-like Mark in Flower Explosion in in Desmodium paniculatum (L.) DC. (Leguminosae) Hiroshi Hiroshi TAKAHASHI Departrilent Departrilent of Biology ，Faculty of Education ，Gifu University ，Gifu ， 501-1193 JAPAN (Received (Received on January 21 ， 2004) Desmodium paniculatum ， which is from North America and is naturalized widely in Japan ，exhibits characteristics typical to explosive f1 owers ，i. e. ， has no retum to original position position in the wings and keel ，spreads a pollen cloud at flower explosion ，rarely has re- visits visits by pollinators ， and is nectarless. The explosion is induced by bee-proboscis inser- tion into into tion the opening between the standard- and the wing-base ， with no force other needed. needed. The f1 0wer possesses marked spots in the basal part of the standard ，that 訂 e very similar similar to the nect 紅 guides common in the nectariferous f1 0wers of Papilionoideae. However ，they 紅 e not guide marks to introduce bees to reward objects ， because it does not not have any reward in the basal p紅t. They appe 征 to function as a guide mark to help bees bees make the f1 0wer explode. Bees can obtain the pollen reward only after insertion of their their proboscis into the opening under the mark. Key words: Desmodium pαniculatum ，explosive f1 ower ，Leguminosae ，nectar guide ，pol- len len guide. Flowers Flowers that provide their pollinators nec- was referred to as a tongue-guide by tar tar as a reward ，especially those with hidden Westerkamp (1997). During this process the nectaries nectaries inside ， usually possess nectar anthers and the stigma in the wing 同 keel com- guides guides (Sprengel 1793). Pollinators can eas- plex come out to touch the bee abdomen. ily ily approach the nectaries by their help. In The pollen grains dusting the abdomen may Papilionoideae Papilionoideae of the family Leguminosae ， be a secondary reward for the pollinators. most nectariferous flowers have nectar Legume species with typical explosive guides guides in the basal part of the standard. Their flowers present a pollination mechanism a nectaηis at the base of the ovary and insects little different from most species with non- can take nectar through holes in the base of explosive flowers. The stamens and pistil in the the filaments with their proboscis (Faegri the flowers of the genera such as Medicago ， and van der P討1 1971). The nect 的 is invisi- Genista and Cytisus are rapidl y released ble ble because it is covered with the basal p紅白 from the wing-keel complex by a visit of of of petals. Bees ， the most common pollina- their pollinators ，and never come back into tors ， land on the wing-keel complex and in- the kee l. After explosion the flowers are sel- sert sert their proboseis into the opening under dom visited again by the pollinators (Kugler the the nectar guide. The opening ，which is be- 1955 ， Faegri and van der P討1 1971 ， Proctor tween the standard- and the wing-base ，intro- and Yeo 1973 ，Lopez et al. 1999). Pollen is duces duces the proboscis of nectar-feeding insects almost the only reward for the pollinators in into into the basal holes of the filament-tube ，and these flowers (Kugler 1955 ， Faegri and van -326- October October 2004 Journal of Japanese Botany Vo l. 79 No. 5 327 der der P討1 1971) ， although Mdeicago spp. se- Macro EF 100 mm lens and a Kenko UV crete crete much nect 訂 (Free 1993 ， Perez-Banon 360 filter) under artificial UV ligh t. The et et al. 2003) and Galloni and Cristofolini color photographs were changed to mono- (2003) (2003) detected glucose at the base of the chrome with a computer using Adobe standard standard in Cystisophyllum and Spartium and Photoshop 6.0. These monochrome photo- at at the filament-tube in Genista. Westerkamp graphs are quite similar to ones made from (1 997) pointed out that the tongue-guide ex 田 monochrome-film (Takahashi unpublished). isted isted even in the papilionate f1 0wers that did Glucose reaction was examined in various not not provide nect 訂， and that bees were con- parts of the f1 0wer with glucose-test paper strained strained into a position suitable for effecting (UROPIECE S made by Toyo Roshi Kaisha pollination. pollination. However ， there are few studies Ltd.) to check nectar exudation. The test addressing addressing how the pollinators know where paper was cut into small stripes (about 1 mm the the anthers ，which 訂 e hidden in the keel- in width) if needed. The paper was softly wing complex ，訂e located and how they ob- pressed against each testable part of the tain tain pollen in the f1 0wers providing pollen as f1 0wer to avoid injuring the tissues. the the main reward. Voucher specimens have been deposited Many species of the genus Desmodium in the herbarium of Gifu University (GIFU). have have explosive f1 0wers (Faegri and van der P討1 1971 ， Ohashi et al. 1981) ， and their ex- Results plosion plosion is .caused by some force additional Flower morphology and blooming move- exerted exerted from outside by the pollinator ment (Faegri (Faegri and van der P詰1 1971). In The calyx is short; the tube is about 1.5 Desmodium ，however ， there are no reports mm long ， the lowermost lobe about 2 mm on what is the most important force to cause long ， the other lobes about 1 mm long. The explosion. explosion. The f1 0wers of Desmodium standard extends a short distance forward at paniculatum paniculatum (L.) DC ，a species from North the base and then curves upw 紅 d at a right America and naturalized widely in Japan ，訂e angle or greater (Fig. 1). The f1 ag part is 6- exploded exploded by a bee- visi t. They have marked 7 mm long and 6-8 mm wide. Two marked spots spots like. a nectar guide in the basal area of green-spots 紅 e found at the standard base the the standard ， although no nectar appe 征 s to (Fig. 2). The wing-keel complex is 6-8 mm be produced. 1 will show the elegant explo 四 long in the non-explored f1 ower. The sion sion of a D. paniculatum f1 0wer by insertion androecium is diadelphous ， but the filaments of of bee proboscis into the opening ， and dis- make a column without any opening between cuss cuss the possible role of the standard spots in the free stamen and the nine fusing stamens the the explosion. in the base (Figs. 5 and 6). The in pistil the column exposes only the distal p紅 t of the Materials Materials and Methods style ，which curves weakly upw 紅 d (Fig. 5). Field Field investigations were made in the There are no nectaries in the f1 ower. plant plant populations of Yanagido ，Sano and Desmodium paniculatum blooms from late Hinakura Hinakura in Gifu City. August to early October. The f1 0wers open Anthers Anthers were counted in the f1 0wers that non-simultaneously in the moming (7-10 had just had a bee visitation and in ones that am). Intact f1 0wers that have not been visited were artificially explored with a needle. by insects extend the wing-keel complex at a UV absorption of the corolla was exam- right angle or greater to the standard (Fig. 3). ined ined through photographs taken by a digital The anthers and stigma 紅 e situated within camera (Canon EOS 10D with a Canon the kee l. After explosion ， the wings and keel 328 328 植物研究雑誌第79 巻第5号 平成16 年 10 月 Figs. Figs. 1 -6. The flower of Desmodium paniculatum. 1: front view of the pre-explored flower. 2: the spots in the lower lower p訂 t of the standard. 3: side view of the pre-explored flowe r. 4: side view of the explored flowe r. 5: side side view of the reproductive column comprising the diadelphous androecium and the pisti l. 6: upside view of of the basal part of the column ，which shows no opening between one free stamen and nine fusing stamens. Bar=2 mm. October October 2004 Journal of Japanese Botany Vo l. 79 No. 5 329 curve curve down at a horizontal angle to the stan- Pollinators and their behavior dard ， but the column ，consisting of the Honeybees (Apis mellifera L.) and solitary stamens stamens and pistil ， stays in the same place ， bees (Nomia punctulata Dalla Torre and resulting resulting in exposure of the whole column Megachile spissula Cockerell) visited the (Fig. (Fig. 4). The down ward curving movement flowers and were effective pollinators. The of of the wings and keel appe 紅 to be due to honeybees approached the flower frontally to recurving recurving in the basal p訂 t of the keel petals. touch the spots in the standard base with the Most pollen grains in the anthers ，which antennae and landed on the wing-keel com- have have already dehisced ， 征 e spread during plex (Fig. 7). Body weight did not cause their their enforced appe 訂 ance through the adher- flower explosion ， probably because the ing ing upper-edge of the keel ， and some anthers peduncles and also the branches with flowers may be pulled off from the filaments (Fig . were thin and flexible (Fig. 8). Explosion oc- 4). 4). The stigmas were apparently receptive curred just after the honeybees pushed their when they appeared. The flowers ， even if not proboscis into the opening between the stan- explored ， began wilting around 3 pm. dard and the wings (Fig.