NEW INSIGHTS INTO THE DEVELOPMENT OF CAMBIAL VARIANTS IN PTERIDOSPERMS

JOSEFINA BODNAR; MARÍA LAURA PIPO; MARISOL BELTRÁN

Congreso; XIV International Palynological Congress - X International Organisation of Palaeobotany Conference; 2016

The stem anatomy of pteridosperms presents a wide variety of secondary vascular patterns, many of which are result of atypical cambial growth (i.e. cambial variants). Their origin and development has been subject of different interpretations since the late nineteenth century. In this contribution, we reinterpret the secondary growth patterns of seed fern families characterized by the presence of cambial variants. We classified the vascular patterns of pteridosperms in the following categories: 1. compound vascular cylinder, constituted by vascular segments surrounded by cortical tissues (Medullosaceae, Amosioxylon), 2. secondary vascular cylinder divided in wedges by large parenchymatous rays (Corystospermaceae), 3. presence of centripetal secondary xylem and phloem (Corystospermaceae), 4. two vascular systems (Medullosaceae, Corystospermaceae, Eoguptioxylon), 5. successive cycles of centrifugal secondary xylem and phloem (Corystospermaceae). By comparison with the stem anatomy and cambial growth of living plants, we could elucidate the types of cambial variants involved in each vascular pattern. The compound vascular cylinder is determined previous to the earliest stages of secondary growth. The interfascicular cambium does not develop, while each primary vascular bundle is surrounded completely by one ring of fascicular cambium. This kind of cambial variant was called multiple cambia, since each ring of fascicular cambium function simultaneously and independently from each other. The secondary vascular cylinder divided in wedges is a result of differential activity of the interfascicular cambium, which only produces parenchyma and sclerenchyma cells. On its behalf, the centripetal secondary vascular tissues are developed by an additional cambium named inverse due to it generates xylem and phloem towards the pith. In the case of two vascular systems, their origin in Eoguptioxylon and Medullosaceae is different from those characterizing the Corystospermaceae. In the former, the two vascular systems are already existent in the primary vascular cylinder; the medullar system has numerous scattered vascular bundles with irregular secondary growth and the peripheral system possess ordered vascular bundles with a uniform secondary growth. In Corystospermaceae, the development of two vascular systems only begins with the secondary growth, considering that the primary vascular cilinder is unique. Finally¸ the successive cycles of centrifugal secondary xylem and phloem are produced by ?successive cambia?. Unlike the ?multiple cambia?, the successive cambia are not active at the same time. Conversely, after functioning for one or more years the first cambial ring cease their activity. Subsequently, a second cambial ring is developed from the innermost cortical parenchyma cells or wood parenchyma.