Free Axial Lobes: An Important Diagnostic Character in Pteridium (Dennstaedtiaceae)

The overall form of the lamina in the bracken fern (Pteridium) is a fractal series in which blade, pinnae, pinnules and pinnulets represent a hierarchy of repeated units of decreasing size. Distinctive lunate or semi-lunate lobes of laminal tissue between the divisions of the blade in an apical zone of some or all of its axes are an important diagnostic feature. These “free lobes” link Pteridium esculentum (G. Forst.) Cockayne subsp. esculentum from Australasia with P. esculentum subsp. arachnoideum (Kaulf.) J.A. Thomson from Central and South America and separate both from P. aquilinum (L.) Kuhn. The development, pattern of distribution on laminal axes and variation of form of the free lobes in P. esculentum subsp. esculentum are described here. Although basic frond architecture as reflected in pinna spacing on the rachis is largely independent of the size of the lamina, the number and pattern of occurrence of free lobes is significantly correlated with the length of axial intervals and hence with frond size and laminal dissection. Attention is drawn to the likelihood that a genetic polymorphism may underlie development of free lobes in P. esculentum subsp. arachnoideum. The presence or absence of free axial lobes may be a useful marker for detection of introgression following hybridisation of P. esculentum and P. aquilinum