ICE FORMATION AND THE DEATH OF PLANT CELLS BY FREEZING

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Abstract
A technique is described for direct microscopical observation of ice formation in living cells. The material examined included stamen hair cells of Zebrina pendula, prothallia of Polypodium aureum, epidermis of red cabbage, and leaves and seeds of wheat. As the temp. was lowered streaming of the protoplasm in the stamen hair cells became slower and gradually ceased. In some cases, Brownian movement was visible for a few seconds after streaming had stopped. Formation of ice occurred first at[long dash]7 to[long dash]8[degree]C in the basal cells and quickly extended to the tip. The ice crystals were granular and compact. Similar granular ice was formed in most cases in the fern prothallia but occasionally needle-like crystals occurred. The order of freezing seemed to agree with the theoretical consideration; ice was formed first in the cytoplasm, then in the vacuole and lastly in the plastids. The changes in the nucleus could not be determined. With thawing, the surface membrane was found to have lost its semipermeable character, allowing droplets of cell sap to collect in the mounting fluid; the cytoplasm hecame granular and disintegrated; the nucleus assumed a glassy appearance. Sucrose solns. had no protective action in preventing killing of red cabbage cells during thawing if ice had been formed within the protoplast. With wheat seeds of varying water contents, there was a close parallelism between the water content of the seed, temp. at which ice formation was observed in the embryo, and their germination after being exposed to temps. below freezing. There was also a close correlation between ice formation and killing by low temps. in leaves of hardened and non-hardened Minhardi wheat plants. The evidence presented strongly suggests that death of plant tissues at low temps. is the result of mechanical injury resulting from ice formation within the cells and that anything which will decrease the possibilities of ice formation within the cytoplasm will thus increase the resistance of the cell to freezing temp.

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