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PALAEOBOTANICAL RESEARCH   GROUP
 
UNIVERSITY  MÜNSTER

THE   RHYNIE  CHERT  AND  ITS  FLORA

 
VIII. ASTEROXYLON  AND  LIFE  STRATEGIES  OF  EARLY  LAND  PLANTS
 
 
Longitudinal section of 
Asteroxylon mackiei
Asteroxylon mackiei, one of the earliest lycopods, is the only plant from the Rhynie Chert which had already small leaf-like structures.  These, however, lacked a vein.  The central stele consists of a very characteristic massive, star-shaped xylem surrounded by phloem (actinostele). The central stele gives of leaf traces which end just at the base of the "leaves".  The development of a relatively complex star-shaped vascular strand in Asteroxylon resulted in a considerable increase of  the exchange surface between the vascular bundle and the cortex.  It also resulted in a much greater stability of the plant, although the number of cells which make up the xylem that provides stability has been kept to a real minimum.  Asteroxylon could be up to 40 cm high and the creeping, naked rhizome formed repeatedly bifurcating root-like organs which could reach up to 20 cm deep.  These latter structures are not true roots because a calyptra, a cap-like structure on the tip of the root, is missing.  The foliated aerial axes bifurcated and borne many additional lateral axes.  The up to 5 mm long leaves were spirally attached.  The kidney-shaped sporangia were scattered, standing on short stalks in the axils of normal "leaves".  In comparison to other Rhynie Chert plants Asteroxylon has a rather complex organization. The development of leaves resulted in a considerable increase of the photosynthetic surface.  The plant was probably better able to regulate humidity by keeping dew drops between the leaves.  This might imply that Asteroxylon was much better able to regulate its water household than the other Rhynie Chert plants.  The stomatal density of Asteroxlon is about ten times higher than that of Aglaophyton.  Unlike other Rhynie Chert plants, Asteroxylon could probably survive in temporarily drier environments.  An overview of the rooting strategies of Rhynie Chert plants is given in the table below: 
 

 
 

 
Although the number of vascular plants in the Rhynie Chert is still limited and the systematical position of some of these species is still unclear, the Rhynie flora ranks among the best known fossil floras.  At least some of the Rhynie Chert plants are known in greater detail than many extant plants.  Even when the vascular Rhynie Chert plants seem at first glance to be not very complex (some may even look superficially similar), detailed studies demonstrate  that soon after the appearance of the first land plants, ecological differentiations expressed in their morphology and anatomy had already resulted in the development of different life strategies. 
Left: A soil with in situ preserved rhizomes of Nothia aphylla; note that all rhizomatic axes are oriented in the same way.


© Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster 
April 2000