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Just a pretty space?
It is always intriguing to find an anatomical feature in an unexpected location. Often such events reflect a malfunction in development. However, apparently misplaced anatomical features may occur in normal development. In the carob tree, Ceratonia siliqua, for example, stomata are found not only in normal location in leaves but also in the seedling root. The formation of these misplaced stomata, as described by Christodoulakis et al. (University of Athens, pp. 22-29) follows the normal pattern seen in leaves. Asymmetric divisions lead to the formation of a pair of reniform guard cells surrounding the stomatal opening. Even the pattern of deposition of cellulose microfibrils resembles that of normal guard cells although rather less in extent. There is also a typical substomatal space beneath the opening. However, these root stomata do not function normally: they remain permanently open even when ABA is applied. Do they lack the signal transduction system present in normal stomata or are they physically incapable of closing? Further, these strange misplaced and nonclosable stomata do not last long. During rapid root growth after seed germination, the elongation rate of the guard cells falls far behind that of the surrounding cells. This places the guard cells under considerable tension and they break at the crown of the curve. This leads to the formation of a long fissure in the epidermis below which is a cavity. The function of this structure is not known. As the authors point out, open stomata and open fissures could provide a pathway for gaseous exchange and the entry of nutrients. However, their distribution along the root is very limited and it is difficult to envisage that their presence would make a significant difference to gas exchange and nutrient uptake. The root stomata thus remain a botanical enigma.
Professor J.A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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