The young plant shows considerable internal differentiation before an apical meristem is organized. Once formed, the apical cell segments slowly. It is triangular in transverse section and segments cut off from its three sides are actively meristematic and soon build up an extensive meristem which enlarges and functions throughout the vegetative phase of the plant's development. Experiments have shown that the apical cell is the main seat of growth control and if it is destroyed another apical cell cannot be regenerated to take over this role. Division of the apical cell followed by division of the whole meristem into two, marks the end of vegetative growth and, the onset of reproduction. Successive divisions of the meristem follow and each new meristem becomes the tip of a fertile branch or receptacle. Eventually meristematic activity at the apices declines and this decline is associated with the termination of conceptacle initiation. As the entire vegetative meristem is used up in reproductive growth, there is no meristem left to continue vegetative grovrth after gamete discharge. Hence reproduction terminates the life span of Himanthalia.