The enucleate, mature sieve elements of Macrocystis possess thick cell walls composed of microfibrils of cellulosic substance impregnated with amorphous substance(s). Sieve plates are also basically microfibrillar with callose deposits. Protoplasmic connections between the sieve tubes and other cells have not been observed.

Freshly cut and fixed sieve tubes reveal a peripheral layer of plastids, vesicles, and endoplasmic reticulum surrounding a large lumen filled with fine granular to fibrillar material interrupted by large masses of electron dense, aggregated slime. Sieve plates with masses of slime and callose constricting pores are numerous.

Liquid nitrogen-frozen, osmium-fixed sieve tubes contain a peripheral layer of plastids, numerous vesicles of many sizes, numerous mitochondria with well-developed cristae, occasional dictyosomes, and endoplasmic reticulum. Masses of slime are absent. The mature sieve tube lumen contains dispersed slime which is continuous through the unobstructed pores of sieve plates.

These fine structural studies indicate many similarities between the sieve tubes of Macrocystis and those of vascular plants. However, the large number of well-preserved organelles in the peripheral layer of Macrocystis sieve elements, constitute a striking difference from most vascular plant sieve tubes so far examined, and may indicate a greater degree of metabolic independence by Macrocystis sieve tubes which lack companion cells.