Pollen Tube Transmitting Tissue: Place of Competition of Male Gametophytes

In angiosperm reproduction, pollen tubes elongate from the stigma through the stylar transmitting tissue to the ovary to deliver the male gametes for fertilization. This article reviews different forms of pollen tube transmitting tracts within the style as well as possibilities for bridging the gap from the transmitting tissue to the ovule. The pollen tube transmitting tissue is the place of possible pollen tube competition. Often the number of pollen grains deposited onto the stigma or stigmata of a flower exceeds the number of ovules in the ovary of the pollinated flower. In the styles of most taxa investigated, a reduction in number of the pollen tubes occurs from the stigma to the ovary (pollen tube attrition): the pollen tubes compete for entry into the embryo sac of the ovules within the ovary. A prediction of the extent of pollen tube competition can be partially equated to the P-S/O ratio. This ratio relates the total number of pollen grains (P) deposited onto the stigmata (S) at the end of the female phase of anthesis (stigma receptivity) to the number of ovules (O) present in the ovary. Some quantitative data on pollen load on the stigma or stigmata and resulting pollen tube competition will be demonstrated especially in the choricarpous gynoecium of Geum rivale (Rosaceae) and in the coenocarpous gynoecium of Cichorium intybus (Asteraceae). Data from this study are compared with data of other taxa obtained from the literature.