Longitudinal and partial surface views of growing cell plates showing involvement of ER in formation of plasmodesmata. ER tubules may be included in openings in the cell plate. Phaseolus vulgaris root tip.
Young wall with plasmodesmata (top); sieve plate elements (left) and companion cells (right). Early stage of callose accumulation visible at ends of a plasmodesma, near differentiating pore site. (Echium angustifolium and E. sabulicola petioles)
Further development of a pore site in longitudinal view. Callose nearly fused at middle lamella, plasmalemma and ER cisternae cover the callose, plasmodesma extends from ER to ER through future pore. (Echium angustifolium petiole)
Left, plugs of callose, penetrated by plasmodesmata, fill the pores. Right, pores open, lined with thin layer of callose, and most are occluded with P-protein, some with starch grains. (Echium wildpreti petioles)
Primary xylem of Phaseolus vulgaris stem, with ring-like and helical secondary walls (close to bottom of slide). Secondary wall surrounds perforations at the two ends of an individual vessel member. Perforations are formed by localized removal of the primary wall.
Removal of vessel end wall with middle part consisting of primary wall material. Around the end wall margin, secondary wall material protects unthickened primary wall, which becomes hydrolyzed and disappear. This stage is shown below: the rim now surrounds a perforation. (Phaseolus vulgaris stem)
Primary wall between contiguous vessel members (right) is partially hydrolyzed between secondary thickening gyres. Noncellulosic wall components removed, cellulose forms a loose network. Hydrolysis affects primary wall adjacent to vessel member. (Capsella bursa pastoris leaf)
Scanning electron microscograph of vessel member from Pelargonium leaf with perforations and pits. Micrograph, courtesy of Professor Peter B. Kaufman and Dr. P Dayanandan, Dept. of Botany, Univ. of Michigan, Ann Arbor, MI.