Browsing by Author "Botha, Christiaan Edward Johannes."

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Studies on the primary phloem of Gomphocarpus physocarpus.
(1974) Botha, Christiaan Edward Johannes.; Bornman, Chris H.
Penetration of the stems and leaves of Gomphocnrpus physocarpus by the aphid, Aphis nerii was studied with light and phase microscopes. Penetration of the epidermis and ground tissue was largely intercellular, that of the phloem tissues partly intercellular and in part intracellular. Of 75 pairs of stylet tips encountered in presumably functional sieve tubes 73 were lodged in sieve tubes of the internal and adaxial phloem. Light and electron microscope investigation of the ontogeny and differentiation of the primary phloem revealed that the internal and adaxial phloem attains a more advanced stage of maturity earlier than the corresponding external and abaxial strands. Furthermore, the protoadaxial phloem sieve tubes remain functional throughout the primary stages of growth, whereas the corresponding abaxial phloem sieve plates are associated with massive deposits of definitive callose during the latter stages of primary growth. The internal and adaxial phloem was found to be capable of independent assimilate translocation in plants which had the external phloem experimentally ringbarked. Translocation in the internal and adaxial phloem of such plants was unaffected by ringbarking. The results are interpreted as indicating that a greater proportion of assimilates is translocated in the internal and adaxial phloem sieve tubes than in the external and abaxial sieve tubes. It is concluded that this governs A. nerii's preference for the internal and adaxial phloem as a prime feeding site.