The multiciliated sperm of the water fern Marsilea vestita was examined with a view to establishing its suitability as an experimental subject. Time‐course experiments revealed spermatid development to be temperature dependent. Sterile techniques were devised for observation of sperm on both a population and an individual basis. Sperm discharge, active and senescing sperm were examined by phase‐contrast microscopy. A regular pattern of senescence was ascertained. This included vacuolation of the cytoplasmic vesicle, loss of motility, and ultimate loss of the helical structure of the sperm coil. Sperm life spans were recorded using motility and O2 uptake as criteria. Sperm populations are active 3–3½ hr at ambient temperature (22–25 C). Individual sperm are active less than 1 hr. Sperm suspensions show a decline in O2 uptake which parallels the loss of motility. Various constituents affecting the life span were investigated. A twofold prolongation of the sperm life span occurred in the presence of 0.1 m sucrose. An ultrastructural examination of the mature sperm was made to aid in assessing its metabolic potential. The sperm shows little ultrastructural differentiation. The cytoplasmic vesicle is predominantly composed of starch‐containing plastids. The main structural components of the sperm coil are a continuous mitochondrial band, an elongate nucleus, and a series of microtubules which separate the basal bodies from the nucleus and mitochondrion. A comparison of ultrastructural features common to Pteridium and Marsilea was made and factors affecting senescence discussed.
