The pollen-stigma interaction in the grasses. 7. Pollen-tube guidance and the regulation of tube number in Zea mays L.

The stigma of Zea mays can support the hydration and germination of considerable numbers of pollen grains, yet, in general, only one tube enters the micropyle to effect fertilisation. The numbers are reduced at various points in the pollen tube pathway, (1) by competition on the receptive trichomes and in the transmitting tracts, (2) by elimination of late-entering tubes at the stigma abscission zone, (3) at a constricted zone of the transmitting tracts in the upper ovary wall, and (4) in the vicinity of the micropyle. The control imposed at the abscission zone is higly effective. The entry of the first few tubes into the ovary wall induces a loss of turgidity of the cells of the zone and a disruption of the vasculation, and thereafter further tubes are irrevocably blocked in the main body of the stigma. The device maximises the period available for pollen capture and yet provides for the strict regulation of the number of tubes ultimately entering the ovary once pollination has taken place. Pollen tubes enter the ovary cavity by breaking through the inner epidermis of the ovary wall, which, like the contiguous inner integument, bears a thin discontinuous cuticle. Further passage is between the inner epidermis and the integument. Growth is irregular, but not wholly random, the tubes orienting themselves in the general direction of the micropyle following lines demarcated by the elongated cells of the epidermis and the integument. Almost throughout the pollen tube pathway, the cells in the vicinity show characteristics suggestive of a secretory function. The significance of this for pollen tube nutrition and guidance is briefly discussed.