SINCE the pericarp in a cereal such as barley is semi-permeable, during the first period of germination the embryo is exposed to a low level of water and oxygen availability (0·1 atm.) and a relatively high carbon dioxide concentration (0·1 atm.) (R. Brown, Ann. Bot., N.S., 93 and 275; 1943). In contrast, when excised embryos are being cultured, as in attempts to elucidate some of the problems of vernalization and kindred phenomena, the young plants are being started under conditions of high water and oxygen availability. Whether grown on water or culture solutions, such isolated embryos always show an immediate drop in dry weight, followed by a slower loss over at least the first twelve hours, suggesting a leaching effect followed by a rather higher rate of respiration than in the embryos of intact grains.