Ultrastructure of the stigma and style of spinach in relation to pollen germination and pollen tube growth

Structural events in the stigma and style of spinach pistils have been investigated in detail with regard to pollination and pollen tube growth. The upper portion of the stigma contains only papillae, papillate cells with a wide central part and a narrow spiral tail part. The lower portion of the stigma also has cylindric parenchymatous cells. In each stigma a central core is formed, initially by the tail parts, later together with the small inner parenchymatous cells. In the style these central cores join and fuse to one central core of transmitting tissue. At the base of the style the central core bends towards the location of the ovular micropyle. In the various portions of the stigma and style the intercellular spaces differ in sizes and also in electron-density of their matrix. Pollen germinates within 10-20 minutes after pollination. Within 7-10 minutes the pollenkitt fuses with the pellicle of the papillar wall. The pollen tube penetrates the pellicle and the eroded cuticle and grows through the outer cell wall layer downwards. Tube growth in the stigma and style is initially intercellular, but following tubes can follow various pathways. In all cases they come out in the space between the carpel and the outer integument. The interactions between pollen/pollen tube and stigma/stylar transmitting tissue are discussed.