Relationships between growth, morphology and wall stress in the stalk of Acetabularia acetabulum

With carbon particles we analyzed patterns of growth in Acetabularia acetab ulum (Lam.) P.C. Silva, a giant unicell famous for classic development stud ies. We focussed on the stalk apex. which generates the stalk, whorls of ha irs, and whorls of gametophores. To gain visual and physical accessibility, we amputated the youngest whorls of hairs and the original apex and perfor med experiments on the apex that regenerated. Video analysis indicated that most growth occurred near the tip of the new apex. Less growth occurred th roughout the cut-interwhorl. We also analyzed cell wall thickness along sta lks cleared of cytoplasm. Correlating growth data to wall morphology sugges ts growth near the apex may be proportional to stress on the cell wall. We propose that turgor-pressure wall stress modulates local apical cell wall g rowth rates. A supplementary model, relating cell wall curvature and growth rate in the cut-interwhorl, characterizes how the stalk's final dimensions and nearly cylindrical shape may arise. See http://faculty.washington.edu/ mandoli/vondassow for supplementary data, analysis, and mathematical append ices. We believe this is the first quantitative description of apex morphog enesis of A. acetabulum.