Development of the intercalary meristem in Chorda filum (Laminariales, Phaeophyceae) and other primitive Laminariales

Development of the intercalary meristem in the terete laminarialean species Chorda filum (L.) Stackhouse was studied in culture using light and transmission electron microscopy as well as by tracing elongation and cell divisions in various parts of the sporophyte. Growth of C. filum sporophytes could be classified into three developmental stages: (i) diffuse growth; (ii) basal meristematic growth; and (iii) intercalary meristematic growth. In the diffuse growth stage, elongation and cell division frequency were almost the same in each cell. In the basal meristematic growth stage, elongation and division of cells became localized in the tissues derived from the meristematic initial cell. Cells of the basal meristematic region contained smaller chloroplasts and many small opaque vesicles. In the intercalary meristematic growth stage, there was further elongation and differentiation of cells originating from the meristematic region, and this became more active in adjacent regions below the meristem than in regions above the meristem, causing the relative position of the intercalary meristem to shift towards the tip of the sporophyte. Meristematic cells of C. filum contained well-developed Golgi vesicles around the nucleus (perinuclear Golgi), many secretion vesicles and many small disk-shaped chloroplasts whose thylakoids were not well developed. Sporophytes of three other terete members of Laminariales, Chorda tomentosa Lyngbye, Pseudochorda nagaii (Tokida) Kawai et Kurogi, and Pseudochorda gracilis Kawai et Nabata, show diffuse growth and basal meristematic growth, but no intercalary meristematic growth. This suggests that the common ancestor of the Pseudochordaceae and Chordaceae had basal meristematic growth, and intercalary meristematic growth evolved more recently in C. filum.