Ultrastructural Localization of Adenosine Triphosphatase Activity in Stigma Cells of Populus lasioearpa and Its Changes During Development Floral Morphology of Populus lasiocarpa Oliv and Its Phylogenetic Position in Populus Fertilization and Embryo Development of Populus lasiocarpa Oliv FORMATION AND DEVELOPMENT OF THE OVULE AND EMBRYO SAC IN POPULUS LASIOCARPA OLIVER HISTOCHEMICAL STUDY OF MICROSPOROGENESIS IN POPULUS LASIOCARPA OLIVER