Proprioception (from Latin proprius: one's own, and capio: I perceive) is the sense in vertebrates that allows us to perceive the position and movement of our own limbs without using sight: thanks to muscle proprioceptors (neuromuscular spindles) and tendon receptors, we know where our arm is even with our eyes closed. Plants have no muscles or proprioceptors in the animal sense, but they do have systems for detecting their own posture in space that guide growth in the appropriate direction. These systems have been called "plant proprioception" in scientific literature, with all the caution the term requires.

Detecting one's own posture: graviproprioceptive systems

The main plant proprioception system is closely linked to gravitropism: plants detect not only the direction of gravity (gravitropism), but also the angle of their own structure relative to the vertical (branch or stem inclination). Statocytes and statoliths: the cells specialized for gravity detection in plants are found in the root columella (root apex) and in the endodermis of the young stem. Starch-containing plastids (amyloplasts rich in starch: called statoliths) settle by gravity to the bottom of the cell. Asymmetric sedimentation of statoliths activates mechanosensitive channels on the lower cell membrane → gravity signal. The angle of inclination: not all parts of the plant grow downward (roots) or upward (stem). The lateral branches of trees grow in different directions (angles other than vertical) and their angle is regulated by a balance between gravitropism (tendency to grow vertically) and phototropism (tendency to grow toward light). The "equilibrium zone" of a branch growing at 45° from the vertical is the result of continuous balancing between these two signals. If the branch is mechanically bent from its equilibrium position, it grows rapidly to return to the normal angle: a system of "postural memory."

Compression wood and tension wood: correcting posture

When a stem or branch is deviated from its vertical position (or from its angular equilibrium position) by external forces (wind, fruit weight, snow, cutting of nearby branches), the plant produces specialized wood that generates internal mechanical forces to restore correct posture. Reaction wood in Gymnosperms (conifers): conifers produce compression wood on the lower side of inclined branches. Compression wood has a different structure from normal wood: rounder cells, more lignified, with high internal compression. The compression force pushes the branch upward. If the branch has been bent downward, the compression wood on the lower side gradually pushes it upward. Reaction wood in Angiosperms (broadleaf trees): broadleaf trees produce tension wood on the upper side of inclined branches. Tension wood has very dense cellulose fibers (G-fibers: fibers with a gelatinous layer of concentrated cellulose) that create a tension force that "pulls" the branch upward. The mechanism for detecting inclination: the signal that triggers reaction wood production is still partly debated. The asymmetric distribution of auxin (the main growth hormone) between the upper and lower side of the inclined branch is the main proposed mechanism. Direct detection of the angle through mechanosensitive channels in cambial cells (the cells that produce wood) likely also contributes.

The "memory of form": programmed vs. adaptive morphogenesis

Plants have a characteristic shape for each species: a beech grows with a shape recognizable as a beech, not as an oak. This "memory of form" is genetically programmed (crown shape, branch angle, branching pattern are genetically controlled) but is also adaptively modifiable in response to the environment (light, wind, competition, damage). The relationship between programmed form and adaptive form: a solitary pine on a windy ridge has a completely different shape from a pine in the same dense forest. But it is still recognizable as a pine: the basic form is genetically programmed, adaptive modifications (prevailing direction of branch growth, crown asymmetry) are the proprioceptive and anemotropic response to the specific environment. "Morphogenetic memory" in plants: meristematic cells (plant stem cells that produce all new tissues) maintain a "memory" of the plant's position and polarity. If a shoot is separated from the parent plant and rooted, the shoot's cells "remember" which pole was the bottom (roots form at the bottom) and which was the top (leaves form at the top). This polarity is maintained by the asymmetric distribution of auxin (which flows from the apex to the roots) and by epigenetic gradients (DNA methylation, histone modifications) that vary between the apical and basal parts of the plant.

Root proprioception: knowing where your roots are

Roots grow in the darkness of the soil and must "know" where they are relative to the surface and relative to other roots of the same plant to avoid competing with themselves (self-avoidance) and to optimize soil coverage. Avoidance of one's own roots (self-avoidance): roots of the same plant avoid each other through perception of their own root exudates. A root that encounters the "scent" of another root from the same plant slows down and deviates: root self-inhibition reduces redundancy in soil coverage. Root growth depth: roots grow downward (positive gravitropism) but at different depths based on oxygen availability (aerotropism: growth toward oxygen), nutrients, and moisture. Roots "know" how deep they are through O2 concentration (which decreases with depth in the soil), temperature (which varies with depth), and probably through hydrostatic pressure signals. Volume of soil explored: a single corn plant explores with its roots a soil volume of approximately 0.5-1 m³. Root architecture (distribution of roots in this volume) is the result of continuous integration of proprioceptive signals (where my roots already are) and environmental stimuli (where nutrients, water, and oxygen are). A mapping of the subsoil through distributed detection systems.

Response gravitropism: how plants return to position

Response gravitropism is the mechanism by which a plant straightens its stem or root after a deviation from equilibrium position. In stems: if a stem is bent horizontally (e.g., by violent hail), within hours it begins to grow upward on the lower side (negative gravitropism: the stem grows away from gravity). Auxins (produced at the apex) redistribute toward the lower side of the inclined stem → the lower side grows faster → the stem curves upward. In roots: the response is opposite (positive gravitropism: roots grow toward gravity). If a root is bent upward (e.g., by encountering a stone), asymmetric redistribution of auxins toward the upper side of the root inhibits growth on the upper side (auxins in roots inhibit growth at concentrations where they stimulate it in the stem) → the lower side grows faster → the root curves back downward. Speed of gravitropic response: in fast herbaceous plants (Arabidopsis, pea), the process of perception-auxin redistribution-gravitropic curvature takes 15-30 minutes. In woody plants, the process is much slower (reaction wood: weeks-months). Memory of equilibrium position: plants "remember" what their equilibrium position was (the typical branch angle) and tend to return to it even when deviated multiple times. This memory is maintained by the stable distribution of auxin signals and probably by epigenetic signals in cambial cells.