Abstract
Certain ultrastructural features of vessel member differentiation were examined in the primary xylem of petiole of bean (Phaseolus vulgaris L.). The cells used had helical secondary wall thickenings and simple perforation plates. The primary cell wall increases in thickness before the helices of secondary wall develop. In a common wall between two vessel members of different ages, theprimary thickening occurs first in the older cell so that for a time the middle lamella is located closer to the younger cell rather than medianly. Apparently the helix is deposited after the primary wall of a given cell reaches maximum thickness. The perforation of the end wall is preceded by primary thickening of the part of the wall that is later removed. The marginal region remains relatively thin and becomes covered with a rim of secondary wall. Vesicles with fibrous content appear near the surface within the end wall shortly before the perforation occurs. A highly vacuolated protoplast with a much enlarged nucleus and numerous organdIes is present during cell wall differentiation. After that process is completed, the protoplast disintegrates and the primary wall bearing the helix is hydrolysed where it is exposed to the cell lumen and, under certain conditions, also under the secondary wall.

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