Ultrastructure and histochemistry of the watermelon stigma.

The ultrastructure and histochemistry of the watermelon stigma were followed from 6 days before flower opening to anthesis. Starch and lipid were present in the immature papilla cells but had largely disappeared from the cells by anthesis, when carbohydrate and lipid were present in the stigma secretion external to the cell wall. Th mature papilla cells had simple wall thickenings and were transfer cells. The wall thickenings were associated with dictyosomes and secretory vesicles. Other characteristics indicating a secretory function for the papilla cells included plastid-ER complexes and close associations between ER and secretory vesicles at the plasmalemma. Granulocrine secretion involving dictyosomes and ER is suggested for the carbohydrate component of the secretion.