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. POLLEN TUBES, root hairs and hyphae of filamentous fungi are known to be predominantly tipgrowing structures (Roelfsen, 1959). Unlike most plant cells where cell wall synthesis and growth commonly occur uniformly over the cell surface, in these structures synthesis and deposition of wall materials and subsequent growth are accelerated in the apical portion of the terminal cell. Ultrastructural examination reveals that these cells share many characteristics including a distinctive stratification of cytoplasmic organelles (Bonnett and Newcomb, 1966; Rosen et al., 1964; Grove, Bracker and Morre, 1970). Numerous vesicles appear in the cytoplasm at the tip and, aside from the presence of ribosomes and occasional profiles of endoplasmic reticulum (ER) and dictyosomes, the growing region of the cell is deficient in mitochondria, ER, dictyosomes and other organelles. In the subapical zone there are dense accumulations of organelles giving the appearance of intense cytoplasmic activity. This unique distribution of organelles has given rise to the thesis that the general pattern of tubular tip-growing cells is dominated by apical accumulations of cytoplasmic vesicles and subapical zones of organelle-rich cytoplasm (Grove et al., 1970). Filamentous fern prothallia and moss protonemata are also tip-growing systems (Roelfsen, 1959; Takahashi, 1961). However, despite considerable study of the ultrastructural characteristics of these organisms there is no indication that their growth conforms to the pattern exhibited by pollen tubes, root hairs, or fungal hyphae. In our previous study of prothallial growth in the fern Dryopteris filix-mas (Stetler and DeMaggio, 1972), stratification of the cytoplasm was not obI Received for publication 22 July 1976; revision accepted 7 October 1976. served. We have also failed to detect obvious stratification and apical vesicle accumulation in the electron micrographs and published descriptions of protonema growth in several species of mosses (Monroe, 1968; Lehmann and Schulz, 1969; Mlodzianowski, 1970; Valanne, 1971; Mueller, 1972). As part of our work on control of oneto two-dimensional plant growth (Stetler and DeMaggio, 1972; DeMaggio and Raghaven, 1973) we studied aspects of the ultrastructural development of the protonema of the moss Dawsonia superba. Our observations on the morphology and distribution of various cytoplasmic components in Dawsonia reported here suggest that stratification of the cytoplasm and accumulation of vesicles at the extreme apex are not necessary for tip growth in moss protonemata. MATERIALS AND METHODS-Spores of Dawsonia superba R. Br. obtained from Professor L. 11. Millener, University of Auckland, were stored at 0 C until used. They were sterilized in a 10 % solution of commercial sodium hypochlorite (Clorox) and transferred to petri dishes containing a nutrient solution (Stetler and DeMaggio, 1972). Germination proceeded in the light at 25 ? 2 C. Collections were made at 24-hr intervals and prepared for electron microscopy. Gametophytes were fixed in 3 % phosphate buffered glutaraldehyde, washed and post-fixed in osmium tetroxide. During the dehydration series, they were stained overnight in uranyl nitrate, then flat embedded in epoxy resin. Gametophytes of the 2-3-cell stage were selected by light microscopy, marked, mounted, and sectioned for observation. When a particular plane of section was required, 2,um sections were taken with a glass knife until the desired section was obtained.