where N t is the number of unimers remaining in the reaction at time t; k is the overall growth rate constant; n a is the number of unimers in the exposed areas per unit perimeter; N 0 is the initial number of unimers; and B is the area contributed per unimer, calculated from the ratio of final platelet area to initial number of unimers (Eq. S6). The area per unimer (B) is considered as a constant, linking the experimentally determined platelet area to the consumption of unimers. Consequently, the evolution of platelet area (A t ) with time (t) can be expressed simply in terms of an effective overall rate constant, k ′ (see SI Methods):