Seeds require an environmental cue to germinate, such as flooding, extended exposure to cold, or a weakened coat from being passed through an animals digestive system. The dry seed absorbs water, which ruptures the seed coat, and activates enzymes that can metabolize food reserves. The radicle emerges first. Orientation of the plane of cell division occurs in part by the action of microtubules that are concentrated in a ring in the cortex. These microtubules disappear before cell division but imprint an array of actin microfilaments that hold the nucleus in a particular orientation.
Expansion occurs when a cell stretches under turgor pressure.
Secreted acids activate enzymes that weaken cross links between cell wall polymers. The cell can only stretch in the direction perpendicular to the polymers of the cell wall. Roots and shoots can detach and become their own plant. These two chapters are concerned with the major evolutionary trends of invertebrates. Become familiar with each of these trends and the example organisms.