Miscellaneous Notes on Pollination and Pollinators

Journal of Natural History ISSN: 0022-2933 (Print) 1464-5262 (Online) Journal homepage: https://www.tandfonline.com/loi/tnah20 Miscellaneous notes on pollination and pollinators P.F. Yeo To cite this article: P.F. Yeo (1972) Miscellaneous notes on pollination and pollinators, Journal of Natural History, 6:6, 667-686, DOI: 10.1080/00222937200770621 To link to this article: https://doi.org/10.1080/00222937200770621 Published online: 17 Feb 2007. Submit your article to this journal Article views: 80 Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tnah20 J. nat. Hist., 1972, 6 : 667-686 Miscellaneous notes on pollination and pollinators P. F. YEO University Botanic Garden, Cambridge Introduction On the grounds that I was a botanist with an interest in insects I was asked many years ago to participate in the writing of a book on pollination. The book is due to appear soon (Proctor and Yeo, in the press). My contribution to it is largely based on previous publications and I have not done any experi- mental work on the subject. However, I have made some observations from time to time and some of these seem to be worth publishing, though most are not suitable for inclusion in the forthcoming book. These, together with some thoughts stimulated by the literature and my continuous contact with British and exotic plants in the Cambridge Botanic Garden, form the very mixed bag of notes presented here. Terminology Guide-marks The phrase 'nectar guide ', which derives from the German Saftmaal (now Saftmal), a term coined by Sprengel (1793, p. 15), is familiar to all who have studied floral biology. However, some nectarless flowers have patterns like nectar guides and the term is then inappropriate. Such flowers are fl'equent in Leguminosae--subfamily Lotoideae--where lines or blotches appear to indicate the entry to flowers which are visited by insects for pollen. Further, it seems likely that some floral markings may be specifically ' pollen guides ' (or ' Pollenmale '--Vogel, 1963, p. 733). I have therefore'adopted the non- committal phrase 'guide-mark '. Lex (1954) has found that scent sources in flowers may also have the form of patterns which have a guiding function, and has used the terms Duftmal (' scent guide ') and Farbmal (' colour guide '). These could be replaced respectively, by ' olfactary guide-marks ' and ' visual guide-marks '. Frequently the scent is produced by the coloured guide-marks. Window-panes A frequent occurrence in insect-trapping flowers (e.g. Araceae, Ascle- piadaceae and Aristolochiaceae) is a zone of more or less colourless translucent tissue bounded by dark-pigmented areas in the chamber where the insects are temporarily imprisoned. This zone evidently has the function of attracting trapped insects towards it. McCann (1943), Van der Pijl (1953) and Faegri & Van der Pijl (1966) have used the phrase 'light-window' for these zones. However, since the function of a window is normally to let in light the expression is somewhat tautologous. A window may be either open or shut, and as we are here dealing with a closed translucent area I propose the use of the phrase 'window-pane '. Bright openings which insects can pass through are also frequent in insect-trapping flowers, often taking the form of lantern-like ]'.N.H. 2 X Published online 17 Feb 2007 668 t ). F. ¥eo structures at the entrance (Vogel 1954); the phrase 'window-pane' is self- evidently not applicable to these, for which the phrase 'window opening' has been used by Faegri & Van der Pijl (1966, pp. 71, 88). Functional morphology of flowers Secondary presentation and protection of pollen The Papilionate flower of Leguminosae, subfamily Lotoideae, has presum- ably evolved from an approximately actinomorphic Caesalpinioid flower, first by developing declinate stamens and pistil, and second by the modification of the two lowest petals to form a keel embracing these organs. This stage of evolution is in fact reached within the Caesalpinioideae (as in Cereis); in the Lotoideae there seems to be no change of principle in the action of the flower, as compared with Cercis, but numerous refinements occur, some involving permanent retention of the anthers within the keel. Declination of stamens and pistil is necessarily associated with an oblique or nearly horizontal floral axis. It occurs chiefly in flowers pollinated by Lepidoptera, which may feed on the wing, and by bees, for which the stamens act as the alighting place instead of the petals. Declination in Lepidoptera- flowers (e.g. Crinum--Voget 1954, p. 97.) may be well-suited to the light brushing of the under-surfaces of a hovering insect which necessarily has only one feeding posture. In bee flowers (for example, Cassia Caesalpinioideae) it must certainly make the flowers less suitable to all Diptera but the most highly flower-adapted, as one does not usually see these insects perching on a collection of horizontally protruding stamens. It would also probably make the collection of pollen by these insects, and possibly also by the more primitive bees, more difficult. Indeed, it could conceivably have begun as a means of protecting pollen from being eaten by any insects. However, most bee- pollinated flowers have compromized on pollen-protection, making their pollen available to bees, if not to other insects, and relying for pollination on grains that have not been collected up into the insects' corbicula. The ' Papilionate 't flower is, in fact, characterized by the protection of the stamens by the petals, and by the return of the petals to a function of supporting the pollinator by their being directed forwards. Once the stamens have become fully protected, a mechanism has to be provided to release the pollen when an insect visits the flower; this consists basically of obliging the insect to push its head into the flower and to depress the wing-petals with its legs, so drawing the keel downwards and in some cases opening the upper edge of the keel. The details of this mechanism were carefully investigated in various genera by Mfiller (1873, 1883). The pushing of the head into the flower is done chiefly in order to get at the nectar, but in some cases it also seems to be a necessary part of the procedure for releasing the pollen, and the insertion of the extended proboscis even into nectarless Papil- ionate flowers is a habit of Bombus lucorum (Brian 1957), but there is no suggesUon in the behaviour of these insects that they are actually testing for the presence of nectar. In such flowers (e.g. Ononis, Lupinus, Sarothamnus) the insect visits are purely for pollen. Only bees seem to be really well adapted ~From Papilionaceae, the name of the Lotoideae at familial rank. Miscellaneous notes on pollination and pollinators 669 to pushing aside barriers of the kind described and such obstacles, which evidently render the flowers more specific to these insects, are found widely in other families with different floral mechanisms, such as Ranunculaceae (Aconitum, Delphinium), Boraginaceae (Symphytum) and Scrophulariaceae (Antirrhinum, Linaria). Thus the raison d'etre of the Papilionate flower appears to be bee-pollination, though in some cases the mechanism has atro- phied even though bees seem to be the pollinators (Amorpha fruticosa), and there has, of course, been adaptive radiation in this great group towards use of other pollen vectors which renders the mechanism irrelevant (e.g. birds in Erythrina ). In many Lotoideae the stamens never protrude from the flower, the pollen being extruded from the tip of the keel by a piston mechanism. Mfiller (1873, 1883) demonstrated a great and, to my mind at present, unaccountable diversity in these extrusion mechanisms. While this diversity is puzzling, it seems safe to conclude that selection acts strongly in favour of such arrange- ments in preference to the protrusion of the anthers themselves. In the flowers with extrusion mechansims the pollen is released into the keel and the anthers themselves shrivel up before the flower opens (fig. 1). stam~ tube FIG. 1. Lotus cornieulatus. (A) Side view of flower with one wing petal, one keel petal and half the upper petal (standard) removed ( × 2½). Pollen is shed into the tip of the keel before the flower opens, and growth thereupon ceases in five of the stamens; the other five continue to grow, keeping pace with the expansion of the flower, while the tips of their filaments swell (B) to form a piston. The piston causes extrusion of pollen during insect visits ( × 3). Other flowers with a basically Papilionate type of mechanism occur in Polygala (Polygalaceae), Hyptis & Coleus (Labiatae) and Collinsia (Scrophu- lariaceae) (Delpino 1871). Whereas the Lotoideae deposit pollen on the under- sides of their pollinators (sternotribic) most Labiatae, in which bee-pollination is also prevalent, deposit pollen on the backs of their visitors (nototribic), and here pollen-protection devices are less well developed. I have seen bees of the, genus Andrena taking pollen by clawing and biting it from the anthers of Lamiunv album, and Dr. M. C. F. Proctor has photographed Syrphid Diptera sucking 670 P.F. Yeo pollen directly from the anthers of the same species; the pollination mechanism, however, is designed for the brushing of the backs of Bombus while the insect's head is deeply inserted into the flower. In spite of such cases, it does seem possible that Diptera might find pollen robbery easier with sternotribic than with nototribic flowers, so that protective devices have become more common in the former. Another factor could be a difference of opportunity for a mechanism to evolve; in sternotribic flowers the weight of the insect on the lower petals virtually creates a mechanism itself, which can then be refined, whereas stamens placed above the visiting insect are not necessarily displaced by the visitor.

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