Botany and Plant Development

Plant development is the result of meristem production (organogenesis) that creates new organs such as leaves, internodes, fruits, rings inside branches, or roots.

Figure 1 Inflorescence types simulated by VisualPlant software (Zhao Xing, LIAMA) : (a) solitary flower, (b) raceme, (c) spike, (d) corymb, (e) capitulum (f) umbel, (g) compound raceme (panicle), (h) raceme of spikes, (i) compound umbel, (j) umbel of racemes, (k) raceme of umbels, and (l) cyme.

Figure 1 Inflorescence types simulated by VisualPlant software (Zhao Xing, LIAMA) : (a) solitary flower, (b) raceme, (c) spike, (d) corymb, (e) capitulum (f) umbel, (g) compound raceme (panicle), (h) raceme of spikes, (i) compound umbel, (j) umbel of racemes, (k) raceme of umbels, and (l) cyme.

Although the structures of inflorescence show regular patterns (Figure 1), which have been described already long ago, the development of shrubs and trees looks more complicated and was not carefully studied before the apparition of plant architecture analysis that allows botanists to understand the link between bud functioning and the resulting three-dimensional (3-D) plant geometry and topology. Through relevant simplifications, the botanists Halle and Oldeman introduced in 1970 the fundamental criteria for classification, giving birth to 23 models of plant architecture (Figure 2). These condensed criteria concern inflorescence position, axis growth pattern and differentiation, and branching patterns. They allow classifying any kind of tree in one of the described models that is supposed to correspond to the stable endogenous developmental pattern of a given plant species.

Each architectural model is an outline of the plant organization. To go further, the so-called architectural unit concept was defined; it refines the plant description and represents the precise and specific expression of the architectural model of a species. According to this concept, axes of a plant may be grouped into a few numbers of categories that are not obligatorily related to branching order, that is, a same type of axis can be found at different precise places in the tree structure according to developmental stages or specific phenomena like acrotony.

This gives a multiscale organization to the plant structure that gives birth to a stack of substructures analog to fractal models, but not necessarily links to autosimilarity. In the last two decades, coupled with precise morphological observations, architectural analyses of several plant species revealed that, under given environmental

Figure 2 Computer simulations of some tree architectural models with VisualPlant software (Zhao Xing, LIAMA) : (a) Corner model, (b) Holttum model, (c) Roux model, (d) Leeuwenberg model, (e) Aubreville model, (f) Scarrone model, (g) Prévost model, (h) Rauh model, (i) Troll model, (j) reiteration of Roux model, (k) reiteration of Rauh model, (l) reiteration of Mangenot model. Modified from Halle F and Oldemann RAA (1970) Essai Sur l'Architecture et la Dynamique de Croissance des Arbres Tropicaux. Paris: Masson.

Figure 2 Computer simulations of some tree architectural models with VisualPlant software (Zhao Xing, LIAMA) : (a) Corner model, (b) Holttum model, (c) Roux model, (d) Leeuwenberg model, (e) Aubreville model, (f) Scarrone model, (g) Prévost model, (h) Rauh model, (i) Troll model, (j) reiteration of Roux model, (k) reiteration of Rauh model, (l) reiteration of Mangenot model. Modified from Halle F and Oldemann RAA (1970) Essai Sur l'Architecture et la Dynamique de Croissance des Arbres Tropicaux. Paris: Masson.

conditions, the structure and features of a particular elementary botanical entity (metamer or phytomer, growth unit, annual shoot, etc.) are predictable and strongly dependent on both: (1) its topological location in the comprehensive architecture of a plant and (2) the onto-genetic stage of the plant. At the level of the whole plant, the 'morphogenetic gradients' notion was defined in order to take into account the intrinsic organization rules of plant structure and was shown to be a powerful concept to explain the observed structure and series of modifications of botanical entities during the ontogeny of any plant species. Features and structure of botanical entities produced by meristems may progressively change along an axis or during plant aging. Powerful botanical notion of 'physiological age of meristems' (PA) describes the state of differentiation of the meristems generating axes and allows clarifying the understanding and interpretation of tree architecture and organization along time (Figure 3). To each PA corresponds a resulting type of generated axis or axis element. Apical meristems can remain in the same state along the branch construction until they mute in another older one. Lateral meristems are usually created with an 'older' PA than the main apical meristems or the same PA may be reproduced by two meristems in case of a reiteration (i.e., duplication of the initial elementary architecture).

Botanists can identify the type of axes and their associated physiological ages even in the complex architecture of mature trees. This leads to a sampling strategy for collecting data and calibrating plant development.

Population Dynamics

Figure 3 Left: Theoretical and diagrammatic representation of tree structure organization according to 'morphogenetic gradients' and 'physiological age of meristems' and (b) mature beech tree architecture. (Left) After Bartheiemy D, Caraglioy, and Costes E (1997) Architecture, gradients morphogeneteques et age physiologique chez lesvegetaux. In: Bouchon J, ReffyedeP, and Barthelemy D (eds.) Modélisation et Simulation de L'architecture des Vegetaux, pp. 81-136. Paris: INRA (Sciences update, Editions). (Right) From Nicolini E (1997) Approcheal morphologique du développement du hetre (Fagus sylvatica L.), 185pp. PhD Thesis, University Montpellier, France.

Figure 3 Left: Theoretical and diagrammatic representation of tree structure organization according to 'morphogenetic gradients' and 'physiological age of meristems' and (b) mature beech tree architecture. (Left) After Bartheiemy D, Caraglioy, and Costes E (1997) Architecture, gradients morphogeneteques et age physiologique chez lesvegetaux. In: Bouchon J, ReffyedeP, and Barthelemy D (eds.) Modélisation et Simulation de L'architecture des Vegetaux, pp. 81-136. Paris: INRA (Sciences update, Editions). (Right) From Nicolini E (1997) Approcheal morphologique du développement du hetre (Fagus sylvatica L.), 185pp. PhD Thesis, University Montpellier, France.

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