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General occurrence of exophylactic and necrophylactic periderms and non-suberized impervious tissues in woody plants Soo, Benjamin Vui Ling

Abstract

The intent of this thesis is to determine whether the new concepts of necrophylactic and exophylactic periderms proposed by Mullick from studies of four species of conifers are generally valid in woody plants, and whether the non-suberized impervious tissue (NIT) recently discovered by him is a prerequisite to the formation of necrophyl actic periderms. Studies of 10 species of conifers with representatives from all coniferous families indigenous to North America and 5 species of deciduous trees by cryofixation technique of Mullic showed the general validity of the necrophylactic and exophylactic periderm concept, the general occurrence of NIT and the invariable presence of NIT prior to necrophylactic periderm formation, establishing NIT as a pre-requisite to necrophylactic periderm format ion. Characterization of periderms in the 15 species showed that, within a species, they could be classified into two categories and that when so categorized, all periderms within a species fulfilled the criteria for either the exophylactic or necrophylactic category, thus establishing the general validity of the concept. Non-suberized impervious tissue (NIT) was found to be present in each species studied. Developmental studies of NIT in Pinus contorta and Larix occidental is, and characterization of NIT in all species, revealed common features in NIT development and features at all sites of its formation in all species studied. Necrophylactic periderm was invariably associated with NIT and was only found after NIT formation. Investigation of the chemical nature of NIT indicated that gymnosperm lignin, or closely related compounds are most likely responsible for the imperviousness. Implications of the role of NIT and necrophylactic periderm formation in pathogenesis as proposed by Mulliek are discussed in relation to the now widely established occurrence of them.

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