Abstract
Summary The formation of ice in the petioles ofSolanum acaule andSolanum tuberosum has been studied by light microscopy and by continuous recordings of latent heat production. Frost hardy material ofS. acaule, when slightly turgor deficient, freezes in two distinct stages. The first short freezing is due to the crystallisation of liquid in the vascular tissues and adjacent intercellular spaces. The second major phase shows the gradual formation of layers of ice in the sub-hypodermal regions of the material after the withdrawal of fluid from the cells of the interior tissues. InS. tuberosum, a frost susceptible plant, the two stages of freezing are less well defined, and are confluent; during the second stage of freezing the ice is laid down at scattered loci through the tissues of the petiole. The behaviour of the petiole ofBrassica oleracea was similar to that ofS. acaule, whilstBegonia rex, an extremely frost sensitive plant, froze similarly toS. tuberosum, but without any indication of a normal double cooling curve.