Pollinia

The trend here is from free unicellular pollen to highly organised waxy pollinia with appendages. In Apostasieae, the anther is of the basic monocotyledonous pattern. The anther is four locular adn the pollen are free. In Cypripedieae also pollen are of unicellular grains but are held together by a viscous fluid. In the rest of the Orchidaceae, pollen are in tetrads, except in a few cases like Cephalanthera of Neottieae and Vanilla or Epidendreae. With the advancement in evolutionary status there are also differences in the organisation of these pollen tetrads. They are held together by means of elastic threads of tapetal origin giving rise to the condition termed mealy or granular and is seen in Neottieae and certain primitive members of Epidendreae. Sometimes the pollinial tetrads are organised into many granular packets, prolongations of which form the caudicle. This is the situation termed sectile and is characteristic of the tribe Orchidieae and the section Spiranthinae of Neottieae. According to Dressler and Dodson this is a specialised development and do not represent any stage in the phyletic line of pollen evolution. In the advanced subtribes of Epidendreae, the pollen tetrads are collected into firm masses called waxy pollinia. In its primitive form, the waxy pollinia are naked but in the advanced subtribes like oncidiinae and Sarcanthinae, they have appendages like the stipe and the viscidium.

From the primitive form of mealy pollen seen in the less advanced subtribes of the Epidendreae like Sobraliinae and Bletiinae, where there are 4 pollinia each divided incompletly halfway between top and bottom, Dressler and Dodson derive all the froms seen in Epidendreae. The simplest derivative of this type is the clavate pollinia as seen in Calanthe. Here there are eight pollinia with four in each anther cell, two of which are smaller than the other two. The second trend is towards the four laterally compressed discoid pollinia as is seen in Epidendrum, Polystachya etc. This is achieved by the loss of the four terminal pollinia. Causicle and viscidia may be present and in some cases even a stripe. Further reduction into two pollinia may occur by fusion, but this is rare. The third line of development is towards superposed pollinia. Subtribes Cyrtopodiinae, Sarcanthinae, Maxillariinae, Oncidiinae etc. are involved in this. Here there is a strong development of stripe and viscidium and reduction by fusion is very frequent.