Fig. 3

In plants, cells are attached to each other by their walls, in the position in which they arise by cell division. As a result, the relative arrangement of cells records sequences of cell division, allowing for reconstruction of developmental processes. The arrangement of cells at the tip of Physcomitrium moss embryos (a, b) reveals growth from an apical cell (images courtesy of C. Jill Harrison); embryo outlined in blue inside the archegonium in a, orange lines emphasize the cells arrangement. The patterning of merophytes formed from derivatives of the apical cell is easily distinguishable in longitudinal sections of Equisetum root (c) and shoot (d) apical meristems and reflects the sequence of divisions of the apical cell; root and stem merophytes traced in orange; root cap merophytes traced in brown in c. Anticlinal (multiplicative) divisions (between arrowheads in e) of vascular cambium initials produce additional files of cells observed in cross sections of secondary tissues (in a Pinus stem). “Doubled” tracheid files (arrowheads in f) are fingerprints that reveal the exact location and timing (measured in wood thickness or growth rings) of symmetric divisions of the cambial initials. Asymmetric divisions of cambial initials initiate rays (arrowhead in g), whose inner ends (arrowhead in h) mark the position and timing of the asymmetric division. Patterns of sporangiophore numbers and sizes along fertile internodes of the Permian equisetalean Cruciaetheca (i) record the basipetal direction of tissue and organ maturation within internodes (j), generated by growth from intercalary meristems; sporangiophores in red, in the image tracing in i and in the diagram in j; internodes gray in i; nodes gray in j; j modified from [47]. Scale bars 20 µm in (a, b); 50 µm in (c, d); 20 µm in (e–h); 1 cm in (i)