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Tyloses from times past
Some modes of fossilization have preserved the cellular structures of the plant in amazing detail. Indeed, the preservation may be so good as to allow direct comparisons with the cells of modern plants and to allow inferences to be drawn about the physiology of fossil species. In some fossils, preserved fungal hyphae are also visible, giving rise to speculations about the possibilities of commensalism or of disease. These features are beautifully illustrated in the paper by Scheckler and Galtier (Blacksburg, Virginia and Montpellier, France, pp. 737-745). This paper is specifically focused on the presence of tyloses. In modern plants, these structures arise when living parenchyma cells grow into the conducting cells of the xylem. This nearly always occurs after the occurrence of an embolism, possibly caused by water stress, in the xylem cells. The function of tyloses is thought to be to block the embolic cells to maintain the efficiency of water conduction. Within the plant kingdom, tyloses occur most frequently among dicot angiosperms but they are known in most other groups of vascular plants. However, they have not been seen previously in the gymnosperm ancestors, the progymnosperms. Thus, the discovery by these authors of tyloses in the wood of Protopitys buchiana is especially significant. This is a progymnosperm tree from the early Carboniferous era; this is therefore not just the first observation of tyloses in a progymnosperm, but it is also temporally, the earliest known occurrence of these structures. Furthermore, the authors were able to show that in this very early wood, the location of tyloses in respect of xylem diameter, distribution of rays and proximity of parenchyma was very similar to that seen in modern dicots. They suggest therefore that back in the early Carboniferous, the physiological function of tyloses was similar to what it is now.
j.a.bryant{at}exeter.ac.uk
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