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How do the directions of cell division and cell expansion affect the shape of a plant organ?

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fern life cycle — Plant development The life cycle of the fern. (1) Clusters (sori) of sporangia (spore cases) grow on the undersurface of mature fern leaves. (2) Released from its spore case, the haploid spore is carried to the ground, where it germinates into a tiny, usually heart-shaped, gametophyte (gamete-producing structure), anchored to the ground by rhizoids (rootlike projections). (3) Under moist conditions, mature sperm are released from the antheridia and swim to the egg-producing archegonia that have formed on the gametophyte's lower surface. (4) When fertilization occurs, a zygote forms and develops into an embryo within the archegonium. (5) The embryo eventually grows larger than the gametophyte and becomes a sporophyte.

The shape of a plant organ is primarily determined by the orientation of cell division planes and the directional nature of cell enlargement.1

If cell division occurs in randomly oriented planes and individual cells expand uniformly, the resulting tissue will grow as a sphere. Conversely, when division planes are oriented regularly or cells expand in a specific direction, the tissue can develop into a variety of complex shapes.1

For example, in a stem, cells in the rib meristem often divide with the cell plate at right angles to the stem axis, creating organized files of cells. When these cells subsequently expand more vertically than horizontally, the stem develops into a cylinder.1

These processes are coordinated by microtubules within the cytoplasm, which help orient the nucleus before division and influence the construction of the cell wall during expansion. By controlling how cellulose is deposited in the wall, microtubules effectively dictate the direction of cell growth.1