Butea monosperma (Lam.) Taub., Dhak' a small to medium sized
deciduous tree, grows gregariously in tropical dry deciduous forests. Besides
providing a timber of commercial importance, its leaves are the good source of
fodder and are locally exploited for making 'plates' and 'donas'. The species
ranks next to ‘kusum' tree (Schleichera oleosa) as a host for the lac insect.
The species which is stable cytologically (2n = 2x = 18) exhibits variation in
floral colour as Indian orange, yellow and white. The cytology, phenology,
flowering pattern, pollination mechanism and breeding system of Indian orange
floral morph is studied presently.
The species remains in deciduous phase for about 45d during
April-May and the flowers appear during leaf fall and deciduous phase. The
trees reach in full bloom during March-April, The zygomorphic and
papilionaceous deep red flowers are present in axillary or terminal 30-60 flowered
compound racemes covered with a soft dark brown velvety hairs. The floral
density is more on the topmost and outer branches as compared to the lower and
inner branches. There is no floral bud dormancy and flora buds develop into
mature flowers in 10-15d. Flower opening and anther dehiscence is synchronous
and occur during night time between 22.00-24.00h. All the anthers in a flower
dehisce synchronously. Stigma which gets receptivity 3-4h before anthesis
remains so for about 48h. Pollen grains are large sized, 3-zonocolporate,
prolate spheroidal and are with reticulate exine. About 47,500-50,000 pollen
grains are produced in a single flower of which nearly 97% are apparently
viable. In vivo test studies reveal that pollen viability is only 4-6% after 14h
blooming, increases to 40% after 20h and reaches to a maximum of 60% after 38h.
Though pollen availability and stigmatic receptivity are synchoronous, selfing
in a flower is generally avoided due to herkogamy. The pollen are transferred
to the stigma through nectar seeking insects like honeybees (Apis mellifera)
and butterflies (Lampides boeticus, Catopsilia pomona) which carry pollen
grains on thorax and wings. On the basis of visiting rate, duration of visit
and pollen load, honeybee seems to be the major pollinator. The landing of
visitors on vexillum results into bending of keel under its weight exposing the
dehiscing anthers leading into nototribic deposition of pollen in honeybees and
stenotribic in butterflies. The species lacks agamospermy but has a better
reproductive success through geitonogamous mode.
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