ACT® · Science: Background Knowledge - Life Science
Plant Development, Growth and Structure
Plant growth is directional and chemically steered: auxin bends stems toward light by stretching cells on the shaded side, stems lengthen only at their tips, and water rises through them by evaporation pulling on a continuous column, no pump required.
A seedling on a windowsill bends toward the glass because light pushes a hormone called auxin to the shaded side of the stem. Auxin makes cells elongate, so the shaded side's cells grow longer than the lit side's. One side stretching more than the other forces the whole stem to curve toward the light, all within hours because it only takes more water and wall-loosening in already-existing cells, not new cell division. This bending response to light has a name, phototropism, and auxin redistribution is the mechanism behind it.
Growth in height works differently from growth in girth. A stem or root only lengthens at its tip, in a zone called the apical meristem, where cells divide and then elongate. That is why a nail driven into a trunk stays at the same height above the ground forever: the trunk isn't stretching in the middle, it's only adding new tissue at the top and new growth rings around its existing girth, driven by a different tissue, the cambium. Height comes from the tip; thickness comes from the sides.
Getting water from root to leaf with no heart or pump relies on the same physics as drinking through a straw, pushed to an extreme. Water evaporates from pores in the leaf, and because water molecules stick to each other, that loss pulls the entire column up behind it, root to tip, under tension. This is called the cohesion-tension mechanism, and it links directly to the plant's plumbing, the xylem, which must be strong enough not to collapse under that pulling force.
Key terms
- Auxin
- A plant hormone that accumulates on the shaded or lower side of a stem and causes those cells to elongate.
- Apical meristem
- The zone of actively dividing cells at a root or shoot tip where lengthwise growth happens.
- Cambium
- A ring of dividing tissue inside a stem that adds new layers of wood and bark, increasing girth.
- Cohesion-tension mechanism
- The process by which evaporation from leaves pulls a continuous, sticky column of water upward through the xylem.
- Xylem
- The plant tissue that transports water and dissolved minerals from roots to leaves.
Watch it explained
- How it worksHow does auxin piling up on the shaded side of a stem make that side grow longer?
- How it worksWalk me through water climbing from a root hair up to a leaf with no pump anywhere in the plant.
- Common mistakeWhy isn't a tree trunk getting taller in the middle as the tree grows?
- Going furtherWhat changes about a root's growth direction when a seed germinates completely upside down?
Check yourself
End of topic.