The septation behaviour of the apical hyphal cell, which solely brings about hyphal elongation, was studied using mycelia grown at different specific growth rates (p) (chemostat and batch cultures). After cell wall staining it was found that both the apical cell (cl) and the adjacent subapical cell (c2) were generally unbranched. Thus, their length (L) could be easily determined.
As the growth rate decreased, Lcl as well as Lc2 decreased, but cells smaller than 5 pm were not observed, even at extremely slow rates of growth (p = 0.06 h-'). The largest cells were observed in rich media, where Lcl attained 41 pm. Since Lc2 represented the length of one of the new-born daughter cells of cl, the distribution curves of Lc2 were used to look for regularity of septation. Especially at slow rates of growth, the curves indicated that in cl septation did not occur randomly. By using Lc2 the interdivision time Tof cl wascalculated. At fast rates of growth it was identical to the earlier determined replication time C, indicating that in mature hyphae septation was coupled to the rounds of DNA replication.
Lcl and Lc2 were used to calculate the length of cl at birth and at the start of septation. It was found that upon septation the appearing daughter cells were differently sized. Depending on the growth rate, the apically situated daughter cell was 1.37 to 1.81 times larger than the subapical daughter (c2). Based on the functional heterogeneity of the sister cells a hypothesis was invented, which could explain the asymmetric septation pattern. It involves the existence of a period S between the determination of the septum site at median position and the actual process of septum formation. The duration of S was calculated, and its correlation to the T and C values at the corresponding growth rates was discussed. Two mechanisms could be distinguished, which were responsible for the immense increase of Lcl at fast rates of growth.