Protonemal Organization and Growth in the Moss Dawsonia Superba: Ultrastructural Characteristics

The elongating, tip cell of the protonema of Dawsonia superba is characterized by a variety of organelles distributed uniformly over the length of the cell. Chloroplasts, vacuoles, mitochondria and dictyosomes are present in the terminal portion of the cell. Dictyosomes are abundant throughout the cell and their production of vesicles is pronounced. Stratification of the cytoplasm and exclusion of organelles from the tip are not observed. The orientation of microtubules and microfibrils follows the pattern observed in filamentous fern prothallia. Microtubules are not observed at the tip of the protonema, but microfibrils are abundant and randomly oriented. While microtubules and microfibrils in lower regions of the apical dome are arranged randomly, more basally they assume an axial arrangement.