TRACHEARY ELEMENTS IN PTERIDOPHYTES

Origin of tracheary elements is related with the migration of plants from aquatic to terrestrial habit. In the Early vascular land plants which survived during Silurian and Devonian Periods the conducting elements were provided with primitive type of lignification i.e. annular and spiral thickenings. Scalariform and pitted tracheids originated independently in different classes of Early Pteridophytes i.e. Trimerophytopsida, Lycopsida and Sphenopsida. Description is given of the structure of tracheids of various kinds of pteridophytic plants beginning with the earliest known extinct pteridophyte Cooksonia to the highly evolved heterosporous plant Azolla. Vessel elements occur in some of taxonomically separated genera e.g. Selaginella, Equisetum, Pteridium, Adiantum, Actiniopteris, Marsilea and Regnellidium. In all, the vessel elements are provided with multiporate terminal perforations. It is believed that in pteridophytes the vessels are polyphyletic in origin and are related more with the function i.e. xeric habit than phylogeny. Relationship among different families of Pteridophytes on the basis of the structure of tracheary elements is discussed.