Aspects related to the nature of stem thickening in monocotyledons have been the subject of many studies. Primary thickening has been attributed to the Primary Thickening Meristem (PTM). According to most authors, it gives rise, besides the adventitious roots, to the vascular tissues and part of the cortex. In other words, it has centripetal and centrifugal activity. For some authors, however, it gives rise only to the vascular system, and for others, only to part of the cortex. However, this work demonstrated that PTM corresponds to the pericycle in the meristematic phase or to the pericycle associated with the endodermis, also with meristematic activity. It was observed that the pericycle was responsible for the formation of the vascular system of the rhizome and of the adventitious roots; the endodermis gave rise to cell layers with radial disposition which comprised the inner portion of the stem cortex, and which corresponded to the region known as the derivatives of the meristematic endodermis (DME). A continuity was also demonstrated between the tissues of the stem and root in species of Scleria Berg. (Cyperaceae).
The epiphytic habitat is potentially one of the most stressful environments for plants, making the effective developmental control in response to external cues critical for epiphyte survival. Because ethylene mediates several abiotic stresses in plants, here, we have examined the ethylene influence in both shoot and root systems of the epiphytic orchid Catasetum fimbriatum. Under controlled conditions, ethylene production was quantified during an entire growth cycle of C. fimbriatum development in vitro, while treatments modulating either ethylene concentration or perception were carried out over the early growth phase of these plants. After treatments, growth measurements and histological features were studied in both shoot and root tissues. Ethylene production showed a decreasing trend over the period of organ elongation; however, it increased considerably when leaves were shed, and a new axillary bud was initiating. The early exposure of young plants to higher concentrations of ethylene triggered morphogenic responses that included root hair formation instead of velamen, and a combination of inhibitory effects (decreases in both stem enlargement and cellular/organ elongation) and inductive effects (increases in leaf and root formation, bud initiation and cellular thickening) on plant growth, which favored biomass allocation to roots. Conversely, inhibition of ethylene perception over the plant growth phase generally resulted in the opposite morphogenic responses. Our data indicate that periodic variations in ethylene concentration and/or sensitivity seem to modulate several developmental features in shoot and root systems of C. fimbriatum which could have adaptive significance during the growing phase of this epiphytic orchid.
ResumoAs microestruturas da epiderme foliar são consideradas de grande valor na taxonomia de vários grupos de Cyperaceae e a deposição da sílica é um dos aspectos considerados de maior relevância. Uma proporção variável das células epidérmicas apresenta corpos silicosos, quase sempre com formato cônico e, relacionados, às regiões costais da folha. Os corpos silicosos podem ter formatos diferentes, como os conglomerados de sílica. Nas espécies aqui analisadas, a sílica é observada formando cones, conglomerados e também, impregnando as paredes das células epidérmicas, inclusive, dos estômatos e dos tricomas. Com base nas informações obtidas conclui-se que os caracteres anatômicos da epiderme das brácteas em espécies de Scleria ocorrentes na Serra do Cipó oferecem alguns subsídios, como o formato dos corpos silicosos, a distribuição dos estômatos e a presença ou ausência de tricomas (curtos e longos), que podem ser utilizados como caracteres diagnósticos para delimitação taxonômica. Palavras-chave: sílica, estômatos, tricomas. AbstractThe leaf epidermis microstructures are considered of great value in several Cyperaceae groups taxonomy and the silica deposition is one of the aspects with great relevance. A variable proportion of the epidermis cells presents silica bodies, almost always with conical shape and, related to the leaf costal areas. The silica bodies assume different shapes as the silica conglomerates. In the species analyzed, the silica forms cones, conglomerates and also, appears impregnating the epidermis cell walls, including stomata and hairs. Based on this paper information, the bracts epidermis in the studied species of Scleria offers some subsidies, like silica bodies shape, stomata distribution and presence or absence of hairs (short and long) that can be used as diagnostic characters in taxonomic delimitation.
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