ABSTRACT
INTRODUCTION Even in a small green plant such as Lemna (3) there are millions of cells, each delimited from its neighbours by a cell wall. The vast majority of these cells are differentiated and, in the vascular plant, often perform specialized functions such as the transportation of water and soluble nutrients in the xylem and phloem (20, 21). All differentiated cells originate from actively dividing meristematic cells (47); these have thin primary walls investing protoplasts with dense cytoplasm (48). These cells are located in the apical (28) and lateral meristems (10, 49) as well as other more localized regions such as meristemoids (50).
