Anatomy of the Medulla of Nereocystis)

Abstract

The medulla of Nereocystis was studied in freshly collected material and the cell types composing it detailed and mapped. In the interest of avoiding implications of a homologous derivation of sieve tubes in angiosperms and sieve filaments in algae, it was desirable to suggest different terminology for the brown algae. The term "sieve filament" as proposed by Esau et al. (1953) was used to designate a filament of cells with simple sieve plates for crosswalls in the medulla of a kelp thallus. "Sieve filament member" indicated a single cell of such a filament. "Trumpet filament" (Smith, 1939) designated a sieve filament that had become attenuated due to mechanical stretching, leaving flared regions where crosswalls were present. A single cell of such a filament was termed a "trumpet filament member". The principal similarities between sieve filaments in giant kelps and sieve tubes in angiosperms are: both types of filaments are produced as a column of cells, both have simple sieve plates (compound sieve plates have not been found in algae), and in both types of filaments the sieve plates may have deposits of callose. Trumpet filaments are apparently produced from sieve filaments by mechanical stretching as the stipe elongates. The bases for this suggestion are the continuous gradations from sieve filaments to trumpet filaments observed in longitudinal sections of young stipe medulla, the occurrence of trumpet filaments centripetal to regions where there are sieve filaments, and a gradation in kelp callose deposits from light in sieve filaments to massive in trumpet filaments. Continual elongation of stipe tissues eventually destroys sieve filaments and would disrupt the translocation stream if kelp callose plugs did not block and shunt photosynthate into younger cells rows. During elongation, associated filaments mechanically support sieve filaments without binding.