Overview of Key Plant Architectural Models

Unbranched trunk – the single axis formed by one apical meristem.

Trunk growth – usually continuous (without pronounced dormancy periods and abrupt leaf change), although in some species (e.g., in female Cycas individuals) rhythmicity may appear.

Branching is absent in the vegetative sphere; branching is observed only in reproduction (generative organs – axillary inflorescences or strobili).

Leaves often large, pinnate or palmate, gathered in an apical rosette (in tree ferns and palms) or arranged spirally (in Carica papaya).

Correlation between trunk diameter and leaf size – “Corner’s rule” is observed: the thicker the trunk, the larger and more complex the leaves (Corner, 1949).

In gymnosperms (cycads) leaves are rigid, pinnate, persisting for several years; in angiosperms leaves can be large and quickly renewed.

Unbranched habit means that such plants lack a natural crown: all photosynthetic organs are concentrated in the apical rosette. For fruit crops (e.g., papaya) this determines the specifics of harvest – fruits set in leaf axils sequentially as the trunk grows, and harvesting often requires ladders or special techniques.

Pruning of such plants is impossible in the traditional sense: removal of the apical meristem (e.g., due to freezing or mechanical damage) leads to the awakening of dormant buds and formation of side shoots, which, however, cannot replace the main trunk in its strict monopodium – most often a bushy or multi‑headed form develops, which is undesirable for commercial fruit growing (Hallé et al., 1978, p. 110). Therefore, in crops following Corner’s model, it is important to preserve the apical bud.

In forestry, single‑stemmed palms and tree ferns are typically first‑canopy trees but with low wind resistance due to the absence of side branches damping trunk sway. Planting them in open windy areas without protective undergrowth is not recommended (Shukla & Ramakrishnan, 1986, p. 34).

Corner’s model has limited relevance for classical temperate fruit pruning, but is important for understanding the growth of tropical fruits (papaya, some Annonaceae) and ornamental palms.

Monopodial orthotropic trunk with clearly expressed rhythmic growth – each increment is separated from the next by a zone of short internodes or scale leaves (traces of a resting apical bud).

Plagiotropic first‑order branches – they grow in the horizontal plane (or with a slight slope), their leaves often have a distichous (two‑ranked) arrangement, in contrast to the spiral arrangement of leaves on the trunk. Branches do not tend to assume a vertical position even when the trunk’s apical bud is removed (stable plagiotropy).

Whorled arrangement of branches: branches develop from the axils of the largest leaves of each rhythmic increment of the trunk, forming distinct whorls (or pseudo‑whorls). The number of branches per tier can vary (2–5 or more) depending on the species.

Photosynthesis – the trunk usually bears small or reduced leaves (often falling early), the main assimilating surface is concentrated on the plagiotropic branches.

Plagiotropy in this model is not irreversible: in experiments, when the trunk is removed, the apical bud of the nearest branch may start orthotropic growth and replace the leader (Hallé et al., 1978, p. 192). However, under natural conditions branches retain a horizontal position.

Wood quality. Trees of Massart’s model have a straight trunk, free of branches (lower tiers die and break off), yielding valuable knot‑free timber – an important quality for sawmilling and plywood production (Shukla & Ramakrishnan, 1986, p. 40). Breeding for increased self‑pruning is carried out in pine, spruce and eucalyptus.

Shelterbelts and wind protection. The horizontal‑tiered crown structure creates a dense “green screen” that effectively reduces wind speed at greater heights. Trees with such architecture are preferred for creating protective shelterbelts in steppe and forest‑steppe zones (provided they are winter‑hardy).

Fruit growing. A straight trunk and clear tiered branching are used in shaping palmette and spindle crowns – the “umbrella” principle is applied for some stone and pome fruits (e.g., peach in spindle form). However, for apple and pear Massart’s model is not dominant (they are closer to Rauh’s model), but the technique of “tiered pruning” itself is borrowed from observations of natural tiered branching.

Shade tolerance under canopy. Horizontal plagiotropic branches allow diffuse light to penetrate to lower layers, enabling the cultivation of shade‑tolerant crops under the canopy of such a forest (medicinal herbs, berry shrubs) – an example of agroforestry (Shukla & Ramakrishnan, 1986, p. 45).

Response to pruning and damage. In trees of Massart’s model, when the apical shoot is removed, dormant buds on the trunk often do not awaken, but one of the lateral branches is activated and begins to grow vertically, distorting the trunk (Hallé et al., 1978, p. 188). In forestry this is undesirable, so in nurseries such trees are kept free of leader damage.

Sympodial trunk – the apical meristem of the main axis after a certain period of growth (usually one rhythmic increment) ceases to function (aborts or transforms into an inflorescence) and is replaced by the nearest lateral bud (or one of the branches of the lower tier). This lateral axis eventually assumes a vertical position and becomes the new leader. As a result, a “bayonet‑like” bend (bayonet joint) forms at the site of the old trunk.

Rhythmic trunk growth – well‑expressed dormancy periods, each increment separated from the next by a zone of shortened internodes or scale leaves.

Branches (lateral axes) – typically plagiotropic by apposition: the apical meristem of the branch continues to grow, but is overtaken by a lateral shoot, which displaces the old tip. As a result, complex sympodial branches with clear articulations (Terminalia-type branching) are formed. Leaves on branches are often arranged in one plane (distichous) or form a spiral with subsequent secondary dorsiventrality.

Modular structure of branches – each branch segment (module) ends with an apical meristem, which then transforms into a shortened shoot (brachyblast) and continues to exist as a leaf rosette. Thanks to this, many long‑lived leaf rosettes accumulate in the crown, increasing the photosynthetic surface (compared to Fagerlind’s model, where branches flower terminally and die) (Hallé et al., 1978, p. 186).

Leaves on trunk and branches are often similar in shape and arrangement (spiral or opposite), but on branches leaves are usually larger. Many species exhibit heterophylly (difference in leaf shape on trunk and branches).

Absence of terminal flowering on branches – flowers are lateral, so branch modules do not die after flowering but continue to function for years.

Tropical fruit growing – many important fruit trees (mango Mangifera indica, avocado Persea americana, sapodilla Manilkara zapota) combine features of Aubréville’s and Rauh’s models in their architecture. Understanding the mechanism of “trunk replacement” allows rejuvenating aging trees by pruning: if the top of the old trunk is removed, one of the side branches below the cut becomes activated and replaces the leader, and the tree continues to grow. This is used in “heading back” pruning of mango.

Forestry – trees of Aubréville’s model often have a non‑straight trunk with traces of multiple tip replacements (“bayonet joints”). This reduces the yield of commercial timber but increases wind resistance and the ability to recover after windbreaks. When establishing forest plantations in windy areas, preference is given to species with a straighter trunk (e.g., Rauh or Massart models), while in protective afforestation (shelterbelts) species with high regenerative capacity are valued.

Temperate horticulture – in some fruit trees (old apple, pear, cherry varieties) when the central leader is damaged, it may be replaced by a side shoot, but this is not the species norm (most cultivars are dominated by Rauh’s model). In breeding for columnar and single‑stemmed habits, forms in which the ability to replace the leader is suppressed are selected.

Pruning and training – for trees prone to Aubréville’s model, it is important not to remove the central leader early without subsequent replacement, as this leads to the formation of a multi‑stemmed bush. When training such trees in orchards, the technique of “pruning to a bud” is used, leaving a strong vertical shoot for replacement.

Agroforestry – in tropical “taungya” systems (mixed cultivation of crops and trees), species with Aubréville’s model (e.g., Terminalia superba) are valued for fast growth and the ability to form a vertical‑tiered crown that thins out after a few years and allows light to reach field crops (Shukla & Ramakrishnan, 1986, p. 45).

Monopodial orthotropic trunk with rhythmic growth – each year (or season) one or several increments are formed, separated by zones of shortened internodes or scale leaves (traces of overwintering buds).

Branches of all orders are orthotropic, i.e., they grow vertically (or at an acute angle upward). Due to this, the crown acquires a characteristic “candelabra‑like” appearance (branches seem to continue the trunk but with gradually decreasing diameter).

Branches are morphologically equivalent to the trunk – they have the same type of growth (monopodial, rhythmic), bear spirally arranged leaves (rarely opposite), and themselves give rise to next‑order branches on the same principle.

Flowers lateral – develop in the axils of leaves of the current or past increments. Terminal flowering does not occur in this model, so the apical meristem never dies due to flowering.

Acrotony – the strongest branches form in the upper part of the increment (closer to the apical bud), which enhances the “candelabra” shape.

Photosynthesis – both trunk and branches bear full‑sized leaves; heterophylly is often observed (e.g., in conifers – young leaves needle‑like, adult leaves scale‑like, but in many angiosperms leaves are uniform).

Most conifers (pines, spruces, firs) and many deciduous trees (oak, maple, ash, chestnut) follow this model (Hallé et al., 1978, p. 226–227).

Training of fruit trees – Rauh’s model is the basis for most crown training systems in temperate orcharding. Apple, pear, sweet cherry and plum are traditionally trained using the modified leader system (in 3–5 tiers), which mimics the natural tiered arrangement of branches. Understanding that branches in such trees are orthotropic and morphologically equivalent to the trunk allows correctly selecting branches for tier establishment and regulating their subordination to the central leader.

Response to pruning – in trees of Rauh’s model, removal of the trunk’s apical bud causes awakening of dormant buds in the upper part, from which strong vertical competitor shoots grow. This property is used in rejuvenation pruning and crown restoration after damage. At the same time, for forming a clear trunk and proper crown it is important to remove competitors at an early age.

Prediction of fruiting – in apple and pear, fruits set mainly on two‑ and three‑year‑old orthotropic branches and on spurs (shortened fruiting shoots). Knowledge of architecture helps in crop load management: the more vertical 2nd‑ and 3rd‑order branches, the higher the potential yield (provided good light exposure).

Forestry – pine, spruce, oak and ash are the main forest‑forming species of Russia. Their architecture (Rauh’s model) provides a straight trunk with good self‑pruning at sufficient stand density. In forest plantations, thinning operations are applied to remove lower, dying branches, improving wood quality.

Breeding for habit – fruit breeders actively work with mutants that have altered architecture (e.g., columnar apples with strongly shortened internodes and absence of lateral branching). Such forms deviate from classical Rauh’s model, approaching Corner’s model (single‑stemmedness) or Leeuwenberg’s model (sympodial branching). Understanding the original model is necessary for identifying and fixing useful mutations.

High‑density orchards – in modern orchards on dwarfing rootstocks, spindle crowns (spindle bush, rudolph) are trained. In these systems, branches are bent to a horizontal position, artificially giving them a plagiotropic character. This suppresses orthotropic branch growth and enhances fruiting wood formation. Thus, knowledge of Rauh’s model allows its modification for agronomic purposes.

Basitonic branching – new shoots (stolons, rhizomes, basal lateral buds) develop only in the lower part of the module, often at ground level or underground. The upper (apical) part of the module never branches.

All axes are equivalent – there is no morphological difference between “trunk” and “branch”: each vertical shoot has the same structure and function (bears a leaf rosette, inflorescences).

Modular structure – the tree consists of repeating modules (individual stems), which can be physiologically autonomous (each develops its own root system). In hapaxanthic species (e.g., banana), the module dies after fruiting but is replaced by a new one from a basal bud.

Absence of secondary thickening (in most monocots) – trunks have constant thickness (increase only by primary growth), limiting their height and branching.

Root system – adventitious roots grow from nodes at the base of each module. In many species (palms, pandanus), stilt roots (support) or pneumatophores (respiratory) develop (Hallé et al., 1978, p. 119).

Photosynthesis – leaves are large, often pinnate or fan‑shaped (in palms) or long ribbon‑like (in bananas, grasses). Leaves are concentrated in the apical rosette of each module.

Formation of a “clump” – over time, a dense cluster of several dozen stems of different ages forms around the mother plant (e.g., in date palm Phoenix dactylifera).

Banana and plantain – understanding Tomlinson’s model is critically important for cultivation technology. After harvest, the main fruiting trunk is cut down, and one (or several) of the suckers (successive shoots) are left for the next cycle. Banana plantations are managed as clonal clumps with continuous module replacement. Breeding aims at varieties with high rate of replacement shoot formation (early “suckering”) and lodging resistance (Hallé et al., 1978, p. 122; Shukla & Ramakrishnan, 1986, p. 44).

Date palm – in plantations, often a single trunk is retained, regularly removing basal suckers (to avoid shading and exhaustion). However, in traditional oases, clump plantings are cultivated, using the clustering ability to increase productivity per unit area.

Sugarcane – a perennial crop that regrows from underground buds after harvest (“ratooning” – the ability to produce new shoots from the rhizome). This is a direct manifestation of Tomlinson’s model. Knowledge of architecture allows calculating optimal plant density and the duration of plantation use before tillering declines.

Cereal grains (wheat, barley, rice, oats) – tillering determines ear productivity and lodging resistance. Breeders select varieties with restricted tillering (for dense sowing) or increased tillering (for sparse sowing and better water use). Agronomists regulate tillering by applying nitrogen fertiliser (nitrogen stimulates side‑shoot formation).

Forage grasses (timothy, cocksfoot, ryegrass) – the ability to regrow repeatedly after mowing or grazing is provided by basal meristems that remain near the soil surface. This allows their use in pasture and haymaking regimes without reseeding for several years.

Oil and coconut palms (though they more often follow Corner’s model – single‑stemmed) – when the apical bud is damaged, in some species basal dormant buds awaken and the plant begins to “clump”, shifting to Tomlinson’s model. This is used for vegetative propagation of valuable forms (though labour‑intensive and unreliable; tissue culture is more common).

Forestry – in bamboos (giant grasses), the “clump” structure (sympodial rhizomes with tillering) determines rapid biomass accumulation and high productivity. Bamboo plantations are managed by selective cutting of old culms to stimulate young shoots.

Monoaxiality (single unbranched trunk) – the vegetative phase is represented by a single orthotropic axis that does not produce lateral vegetative shoots. Lateral meristems are either absent or remain deeply dormant throughout life.

Terminal inflorescence – after a multi‑year (sometimes very long, up to 40–80 years) vegetative phase, the apical meristem transforms into a giant inflorescence (panicle, spadix, raceme). All accumulated biomass is mobilised for flowering and fruiting.

Monocarpy (death after fruiting) – after seed maturation, the plant dies completely. This distinguishes Holttum’s model from all others, where the plant remains alive after flowering.

Continuous growth (most often) – during the vegetative phase, trunk growth is continuous, without pronounced dormancy periods, although in some species (e.g., cycads, but they belong to other models) rhythmicity may occur. In palms of Holttum’s model (e.g., Corypha), leaves are large, spirally arranged, forming an apical rosette.

Leaves – usually very large (in Raphia regalis leaves reach 25 m in length – a record for plants), pinnate or fan‑shaped. Leaves are concentrated in the upper part of the trunk, gradually dying from below.

Root system – adventitious roots, often with aerial (support) roots in tall palms. After flowering, the roots also die.

Size and longevity – can reach enormous sizes (height up to 20–30 m) and live for decades (in Corypha umbraculifera up to 40–80 years), contrasting with annual monocarpic herbs.

Agave – an important technical and food plant. Agave fibre (sisal) is obtained from the leaves before flowering. After flowering the plant dies, so agave plantations are renewed via offsets (vegetative propagation) that grow from the base of the mother plant (although basal branching is not characteristic of strict Holttum’s model, agaves often produce lateral offsets, bringing them closer to Tomlinson’s model, but the main shoot remains monocarpic). For tequila production, the core of Agave tequilana is used – it is cut before flowering to stop inflorescence development and accumulate sugars.

Monocarpic palms (raffia, sago palm, talipot) are used by local populations for starch (sago palm), fibre (raffia), construction material and palm wine (by tapping the inflorescence). Knowledge of the model allows planning harvest: starch is maximum just before flowering, so palms are felled before inflorescence formation begins.

Ensete (“false banana”) – an important food crop in Ethiopia and parts of East Africa. A starchy mass (“kocho”) is obtained from the pseudostem and underground parts. The plant is monocarpic, so after harvest (or after flowering if allowed) it dies. Ensete is propagated by suckers.

Ornamental horticulture – some monocarpic plants (agaves, talipot palm, some lobelias) are grown as exotic perennials that behave like annuals in terms of flowering. Understanding that the plant will die after flowering allows planning landscaping (e.g., removing the flower stalk from agaves to extend the plant’s life). In the talipot palm, flowering is a unique event attracting visitors to botanical gardens.

Forestry – in monocarpic trees (a rare strategy), the wood loses commercial value after flowering (the trunk becomes soft and brittle). Therefore, such species are not used in industrial logging.

Breeding – for monocarpic crops, breeding for late flowering (to increase the period of biomass accumulation) and for vegetative propagation (offsets) to maintain plantations is important. Work is also underway to obtain sterile hybrids that do not flower and do not die, but such forms lose the ability for seed reproduction.

Transitional forms – in some species (e.g., mango, avocado) architecture combines traits of different models (Rauh and Aubréville). Accurate diagnosis requires observing young plants under optimal conditions.

Change of model with age – in some trees (e.g., oak, beech) in youth Rauh’s model dominates (clear tiers, candelabra branching), while in old age (especially after damage) features of Aubréville’s model (sympodial trunk replacement) may appear.

Breeding significance – modern fruit cultivars may be deliberately deviated from the natural model (columnar apples, weeping forms). However, knowledge of the original wild model is necessary for understanding the genetic mechanisms of habit.