6.Jl APPENDIX VI.J TRANSPIRATION OF PLANTS Transpiration is the phenomenon of water evaporation from the surface of the mesophyll cells of plant leaves; water diffuses out mainly through the stomates (6.Jl). Factors which affect the rate of transportation include leaf area, structure, orientation, water content, root-shoot ratio, humidity of the air, temperature, wind, vital activity, effects of sprays and dusts, water supply, but most of all, light (6.J2). Transpiration peaks about three in the afternoon, at about 45 mg/cm 2 leaf area per hour, and drops to near zero at night (6.J3). Water absorption lags behind transpiration during the day, but increases at night to replenish the necessary percent of water content. The weight of water that a plant uses while producing one pound of dry matter, expressed as the ratio of water used per dry matter produced is known as the transpiration ratio. A table of typical values follows (6.Jl): Species TransEiration Ratio Pine 50 Corn 350 Wheat 450 Apple 500 Alfalfa 800 The average water content of herbaceous plants is 80%. Some plants can lose in one day an amount equal to their total water content (6.Jl). Assuming these maxima are the case, since the plant biomass per person per day is 86 kg, 68.8 kg of water is transpired per person per day. For the 1,000 person colony, this means 68,800 kg of water is transpired per day, mainly during the 18 hours of sunlight, or 3,822 kg/hour of sunlight. The agricultural area is provided with 400 watts/m 2 , or 619 cal/cm 2 -day of sunlight. Since clouds tend to form when the relative humidity is 75% or higher, and the colony's relative
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