Histocytology Observation on the Somatic Embryogenesis in Ananas comosus Callus Histocytology Observation on the Somatic Embryogenesis in Ananas comosus Callus Healing Responses of Tube Grafted Tomato Plug Seedlings under Different Night Temperatures Histocytology of Interaction between Phytophthora infestans and Potato Cultivar with Horizontal Resistance ORIGIN OF FLORAL BUDS IN TOBACCO PEDICELS CULTURED IN VITRO