Yucca baccata and Yucca elata are Chihuahuan desert plants which develop massive inflorescence stalks, large flowers and heavy fruits. A field study on the NMSU College Ranch found an average Y. baccata invests over 250 g of biomass (oven dry) into these reproductive structures, with about half of this in fruits. Its fruits are strongly affected by the yucca moth whose larvae develop inside the fruit and consume the maturing seeds. A comparative laboratory study found the optimum net photosynthesis rate of Y. baccata leaves below 25 C, whereas Y. elata leaves had an optimum above 30 C, assuming equivalent moisture and irradiance levels. These physiological differences correlate well with their observed patterns of vegetative and reproductive growth. Y. baccata grows better in cooler seasons and sites than Y. elata. A computer simulation study of reproductive and vegetative growth in Y. baccata indicated that an average-sized plant needs about 10-12% of its dry biomass as stored carbohydrates in order to produce an average-sized inflorescence with flowers and fruits. A lesser amount resulted in depletion of stored energy to less than 1%. Depending on environmental conditions, the simulation indicated that about three years were required to rebuild energy to the level needed for another reproductive effort.