Mitoses in thermodormant lettuce seeds with reference to histological location, localized expansion, and seed storage

Seeds from one lot of "New York" lettuce were sown in each of 5 successive years to compare the effect of duration of storage on the capacity for mitosis in thermodormant seeds with the effect of storage on the capacity for germination under conditions favorable for germination. Whereas the capacity for mitosis in absence of germination increased as a function of duration of seed storage, the capacity for germination itself decreased steadily over the period studied. Thus, the apparent "deterioration" of stored seed, as measured by decreased germination, does not necessarily indicate a general deterioration of all cytologic processes. Histological study of thermodormant lettuce seeds demonstrated that in some seeds mitosis and cytokinesis can occur without either overall or localized expansion of the embryo-axis. Although localized expansion occurred in some seeds, there was no expansion of the embryo-axis as a whole in any of the seeds, the localized expansion being accompanied always by localized compression elsewhere in the axis. Restraint of the embryo by the endosperm, which remained intact, could account for the prevention of overall expansion of those embryos in which localized expansion and compression occurred. Mitoses in such embryos occurred both within and outside the regions of localized expansion. Thus, even in those embryos with localized expansion, mitotic activity is not necessarily correlated with expansion. Mitoses occurred in each of the primary tissue meristems of the hypocotyl-radicle, mainly within the apical 0.5 mm of the radicle. In this regard, thermodormant embryos resemble growing roots of lettuce seedlings. These findings demonstrate that mitotic activity and localized expansion, either separately or together, can occur in the embryo without germination.