Studies on the primary phloem of Gomphocarpus physocarpus.

Abstract

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.