The Plant's Circulatory System: From Leaf to Root, and Even to the Neighbors
Plant Hormones
Plant hormones—phytohormones—are the chemical signals that tell a seed when to sprout, a shoot when to stretch, a root when to branch, and a tomato when to turn red. They don’t ride a bloodstream the way animal hormones do. Some move cell to cell in a set direction, some travel in the xylem and phloem with water and sugar, and one of them is a gas that drifts through the air spaces inside the plant. Once you know what each one does and how the ratios shift, pruning shears and a bottle of rooting hormone stop being guesswork.
A bit of history
The first one found was auxin. In the 1880s Charles Darwin and his son Francis covered the tips of canary-grass seedlings and watched them stop bending toward light. Covering the base did nothing. Something at the tip was directing growth farther down the stem. Decades later Frits Went used agar blocks to catch whatever was leaking out of a cut tip and transfer it; the substance moved and the response followed. He named it auxin, from the Greek for “to grow.” That same logic—separate the source from the response and see what travels—still guides a lot of the work.
The main players
Auxins. The primary one is indole-3-acetic acid (IAA), made mostly in the shoot tip and young leaves. It drives cell elongation, root formation, and gravitropism: tip a stem on its side and auxin piles up on the lower face, cells there stretch, and the shoot curves back up. It is also the hormone that keeps lateral buds quiet until you cut the tip.
Gibberellins. These stretch stems, help seeds germinate, and in some species trigger flowering. Produced in tips, young leaves, and developing seeds. That long internodes you see when a plant is reaching hard for light is often gibberellin at work.
Cytokinins. Made mainly in root tips. They push cell division and wake up lateral buds. The important part is the ratio with auxin: high auxin relative to cytokinin keeps buds dormant; tip the balance the other way and the buds break.
Abscisic acid (ABA). The stress hormone. It closes stomata when water is short or cold hits, and it keeps mature seeds dormant until conditions are actually good for germination.
Ethylene. A gas. It ripens fruit, ages flowers, and triggers abscission—the drop of leaves, flowers, or fruit. One ripening or damaged apple really can speed the rest of the basket; the gas moves.
A few newer classes matter for the work we do here. Strigolactones help suppress branching (another piece of apical dominance) and encourage mycorrhizal partnerships at the roots. Brassinosteroids support cell division and stress tolerance. Jasmonates and salicylic acid handle defense—one after insect damage, the other against pathogens. We don’t spray those last ones; they just explain why a chewed leaf or a fungal attack changes the whole plant’s behavior.
How they move
Not every hormone travels the same route. Auxin uses polar transport—an energy-requiring, cell-to-cell path that runs mostly downward from the tip. That direction is why apical dominance works. Cytokinins coming up from the roots and ABA moving during drought ride the plumbing: xylem upward with water and minerals, phloem with sugars from the leaves. Both of those tissues are produced by the vascular cambium—the same thin layer that has to line up for a graft to take and the same layer that has to make callus at the base of a cutting before roots can form. That anatomy is its own subject and belongs with the companion pieces on this site.
Apical dominance—what actually happens when you top a plant
Auxin from the shoot tip flows down and holds the axillary buds in check. Cytokinin from the roots is already there, waiting. Cut the tip and the auxin source disappears; the ratio flips and the buds below the cut start growing.
Two different jobs get mixed up under the word “topping.”
On a young plant you want more structure—pepper, basil, or a fruit tree in its formative years—you pinch or make a heading cut so several laterals break at once. More branches mean more sites for flowers and fruit later. That is the mechanism behind almost all early training of trees and shrubs.
On a mature, fruiting plant late in the season the goal is different. Top an indeterminate tomato in September here in zone 5 and you stop it from pouring energy into new vegetative tips and flowers that will never ripen before frost. The remaining sugars go into sizing and finishing the fruit already set. Same hormone shift, different purpose.
Putting it to work
Pruning and pinching is the most common lever we have. You are not adding hormones; you are removing the auxin that was suppressing the ones already present.
Commercial gibberellin (GA3) can stretch cut-flower stems or enlarge seedless grapes; it is used in malting barley for uniform germination. Ethylene or ethephon is how bananas and tomatoes are brought to market color after harvest—the same chemistry as the paper-bag-and-ripe-banana trick at home. ABA products can help hold seed dormancy for longer storage, though they are less common on the home-garden shelf.
Rooting hormone is the one we reach for most often in the nursery. Almost every commercial powder or gel is a synthetic auxin—IBA or NAA rather than natural IAA, because the synthetics are more stable and more effective at starting roots on a fresh cutting. Dip the base, stick it, and success rates climb, especially on the harder woody plants.
A word of caution
These products are useful, but several synthetic auxins at higher rates are regulated as growth regulators or herbicides. Follow the label. More is not better; over-application is one of the fastest ways to damage the very plant you were trying to help.
The practical takeaway is simple. Know which hormone is driving the response you want—branching, rooting, ripening, or holding dormancy—then use the cut, the dip, or the timing that shifts the balance in that direction. Here at the Sanctuary that usually means a sharp pair of pruners in spring and early summer for structure, rooting hormone on cuttings when we take them, and a late-season heading cut on tomatoes so the fruit we already have finishes before the first hard freeze.
Next step for this stretch of September: walk the tomato rows and any young fruit trees that still have soft tips pushing. Decide which ones need the growth stopped so energy stays in the crop, make the cut clean, and leave the rest alone.
Further reading and sources
If you want the science behind this piece, start here. These are the places I would send a grower who wants more than a nursery label.
Clear overviews
Plant Hormones — Biology LibreTexts (Coalinga College plant science) — table of the five classic hormones, what each does, and how growers use the synthetic forms (IBA, NAA, GA3, ethephon).
Auxin — Biology LibreTexts (Kimball) — phototropism, gravitropism, apical dominance, and root initiation in plain language.
History of auxin
Darwin, C., and F. Darwin. The Power of Movement in Plants. 1880. The tip-cap experiments that started this whole line of work.
Went, F. W. 1926–1928. Agar-block isolation of the mobile signal later named auxin. Good modern retelling: Understanding phototropism: from Darwin to today (Journal of Experimental Botany).
Odyssey of Auxin — how polar transport was demonstrated after Went.
Apical dominance, ratios, and newer hormones
Lessons from a century of apical dominance research (Journal of Experimental Botany, 2023) — auxin, cytokinin, and why strigolactones belong in the same conversation.
Developing a model of plant hormone interactions — the five classics as a network, not five separate switches.
Vilain et al. 2026. Towards an integrated molecular understanding of plant hormones (Journal of Integrative Plant Biology) — current map of the nine classes, including brassinosteroids, jasmonates, salicylates, and strigolactones.
Rooting hormone in practice
Plant Propagation by Stem Cuttings — NC State Extension — why IBA and NAA are used on cuttings, and how to handle woody vs. softwood material.
USDA AMS technical review of IBA (used by Extension when growers ask about treated cuttings): Indole-3-Butyric Acid TR (PDF).
On this site
