Young et al. ([ref.52]) assumed that water is present on Mars, at least in microenvironments, and that nutrients would be available. The primary objective of their experiments was to determine the likelihood of contaminating Mars with Earth organisms should a space probe from Earth encounter an optimum microenvironment in terms of water and nutrients. The experiments used bacteria in liquid nutrient media. The environment consisted of a carbon dioxide-nitrogen atmosphere, and the temperature cycling was -70° to +25° C, with a maximum time above freezing of 4½ hours. _Aerobacter aerogenes_ and _Pseudomonas sp._ grew in nutrient medium under Martian freezing and thawing cycles. Atmospheric pressure was not a significant factor in the growth of bacteria under these conditions. Silverman et al. ([ref.47]) studied bacteria and a fungus under extreme—but not "Martian"—conditions. Spores of five test organisms (_B. subtilis_ var. _niger_, _B. megaterium_, _B. stearothermophilus_, _Clostridium sporogenes,_ and _Aspergillus niger_) and soils were exposed while under ultrahigh vacuum to temperatures of from -190° to +170° C for 4 to 5 days. Up to 25° C there was no loss in viability; at higher temperatures, differences in resistivity were observed. At 88° C, only _B. subtilis_ and _A. niger_ survived in appreciable numbers; at 107° C, only _A. niger_ spores survived; none were recoverable after exposure to 120° C. _B. subtilis_ survived at atmospheric pressure and 90° C for 5 days, but none of the other spores were viable alter 2 days. Four groups of soil organisms (mesophilic, aerobic, and anaerobic bacteria, molds, and actinomycetes) were similarly tested in the vacuum chamber. From one sample only actinomycetes survived 120° C, while one other soil sample yielded viable bacteria after exposure to 170° C. Several organisms resisted 120° C in ultrahigh vacuum for 4 to 5 days. When irradiated with gamma rays from a cobalt 60 source, differences were observed between vacuum-dried spores irradiated while under vacuum and those exposed to air immediately before irradiation. A reduction of from one-third to one-ninth of the viability of spores irradiated in vacuum occurred with vacuum-treated spores irradiated in air. Siegel et al. ([ref.73]), in approximate simulations of Martian environments, studied tolerances of certain seed plants, such as cucumbers, corn, and winter rye, to low temperatures and lowered oxygen tensions. Lowered oxygen tensions enhanced the resistance of seedlings, particularly cucumber and rye to freezing, and lowered the minimum temperature required for germination. Germination of seeds in the absence of liquid water has also been studied. In this case, seeds of xerophytes have been suspended in air at 75-mm Hg pressure above water. The air was thus saturated. Germination was slow but did occur.