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5 Fig. 1.1. Reconstruction of a Late Triassic forest in Greenland , showing the tufted appearance of Mesozoic forest tree crowns. Image courtesy of Marlene Donnelly and Jennifer McElwain.© by Marlene Hill Donnelly. ARchITecTuRAl InnovATIon And develoPMenTAl conTRols In soMe MesozoIc GyMnosPeRMs, oR, Why do The leAF cRoWns In MesozoIc FoResTs look TuFTed? Ian sussex, nancy kerk, and carole T. Gee The concept of plant architecture was first formalized by Hallé, Oldeman , and Tomlinson in 1978 and was based on patterns observed in forestforming trees. Plant architecture is dependent primarily on the function of meristems and the growth patterns of shoots and roots. There has been an evolutionary progression in architecture throughout the history of plant life, and Mesozoic gymnosperms occupy an intermediate position between the earliest and the latest patterns. We shall discuss the molecular , genetic, and physiologic bases of advanced plant patterns and then deduce which changes have occurred in evolutionary terms that make the shoot architecture of Mesozoic gymnosperms different from modern forest-forming trees. This may be largely the result of an evolutionary progression in shoot growth patterns, such as internodal extension, which was, in the Mesozoic, likely a hormone-regulated process and possibly under the developmental control of polar auxin flow, as it is today. Many Mesozoic gymnosperms occupy a unique place in plant evolution , demonstrating as they do the transitions between the relatively simple morphologies of the ferns, lycophytes, and early seed ferns to the much more complex morphologies, including floral morphologies, of the angiosperms. In this era, which spanned some 180 million years, the most common plants were cycads, bennettitaleans (also known as cycadeoids), conifers, and ginkgophytes. Plants of these groups were the dominant members of Mesozoic forests, and they shared a common feature in possessing stems with little or very restricted internodal extension , although this was less a feature of the Ginkgoales. The consequence of this growth habit was that Mesozoic forests were composed of several major plant groups with tree crowns that were remarkably tufted and with stems that were extensively covered and often protected by scaly leaf bases (Fig. 1.1; Plate 1). They looked dramatically different from the forests, particularly the tropical forests, of the present. They also differed from Paleozoic coal swamp forests in which only one plant group—the arborescent lycophytes—had tufted crowns. What were the changes in Introduction 1 Sussex, Kerk, and Gee 6 growth characteristics that underlay the transition from the tuft-crowned Mesozoic gymnosperms to the modern trees of the present-day forests? The huge differences in morphology of the vascular plants make this seem like a daunting task. However, examination of the architecture of plants provides a method for categorizing and systematizing the events of this transition. The concept of plant architecture, which was first formalized by Hallé et al. (1978), is based primarily on earlier descriptions of tropical forest trees. Most attention has been paid to shoot architecture, although there are some descriptions of root architecture (Atger and Edelin 1994; Osmont et al. 2007). Because of the paucity of preserved root systems of Mesozoic plants, we shall not consider these in this analysis. The architectural analysis of shoot systems as originally proposed by Hallé et al. (1978) conceived the plant as conforming through its growth cycle to an architectural model that is an abstraction for the deterministic genetic blueprint on which the construction of the plant is based. Under ideal conditions of growth, the development and architecture of the plant would conform strictly to the model. Hallé et al. identified 23 different architectures that described essentially all of the vascular plant growth habits, as well as those of 24 fossil plants, including several members of the Bennettitales, Cycadales, Cordaitales, and Coniferales. The fact that plants of the same species differ in appearance was attributed to interactions between the environment and the development of the individual organism. They termed these changes opportunistic, so that the developmental morphology of a plant represented a balance between deterministic, genetic and opportunistic, environmental effects (Hallé 1999). The architectural models were considered to be indicative of the growth and developmental processes of a particular phase of the plant’s life cycle, and they found that as a plant passed through various phases of its life cycle, such as transitions involved in phase change, aging, heteroblastic development, or flowering, the model might change to another one indicative of different growth processes. Vascular plants are modular organisms that develop by the repetition of certain basic entities, albeit with some...

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