Experiments were conducted on the cellular responses of monocotyledonous and dicotyledonous plants to sublethal and lethal temperatures. The study had four objectives: I. to compare temperatures that develop in stems and leaves of plants under flame, II. to determine the effect of heat on conduction in the phloem (of stems), III. to investigate some cellular responses to sublethal and lethal temperature treatments, and IV. to determine the effects of lethal temperatures on respiration and on the Hill reaction.

When stems and leaves of plants were exposed to a flame at 1900°F. for 130 milliseconds, there was a reduction in amount of heat that penetrated into stems of monocotyledon plants as compared to that penetrating into stems of dicotyledon plants. This difference in penetration appeared to be related to the morphology of the stem at the time of measurement. There were no differences between species tested in leaf tissue injury as indicated by degree-second summations. Heat penetrated less into stems than into leaves, and less into older plants than into younger plants. The temperature was higher at the stem periphery than near the stem center.

C¹⁴ activity was greatly reduced in the roots of flame treated soybean plants when compared to the non-flamed plants 24 hours after the photosynthesizing parts of the plants had been exposed to C¹⁴O₂. The function of the sieve elements in conduction of the C¹⁴ photosynthates may have been temporarily blocked by the heat treatment.

Loss of chlorophyll and swelling of chloroplasts were observed in cells of elodea leaves exposed to sublethal temperature treatments.

At the thermal death point of leaf cells of elodea, corn, soybeans, pigweed, and yellow foxtail there appears to be a disorganization of the tonoplasts, plasma membrane, chloroplast membranes, and an irreversible stoppage of cytoplasmic streaming in leaf cells of elodea. Approximately 40 percent of the cells in elodea leaves show criteria of cell death at a temperature treatment which results in necrotic leaf tissue. Plasmolysis was observed in leaf cells at the lethal temperature treatments but did not appear to be a primary event at the cellular thermal death point.

Changes in leaf cells of elodea after death were observed on certain days after the leaves had been exposed to lethal temperature treatments. In plasmolyzed cells of elodea leaves, observed immediately after treatment, there appeared to be chlorophyll in the cytoplasms and the chloroplasts appeared shrunken. The chloroplasts stained a dark blue in these cells (when stained with toluidine blue) indicating a disorganization of the chloroplasts. Cells observed two days after lethal temperature treatments exhibited slightly bleached chloroplasts, the cytoplasms appeared to be coagulated, and the plasma membranes appeared to be breaking down. The nucleus appeared to be still intact in some of the cells. Cells observed four days after lethal temperature treatments exhibited bleached chloroplasts with only remnants of the chloroplasts being apparent in some cells. The protoplasm in all the cells show characteristics of coagulation and the plasma membrane and the nuclear membrane appeared to be broken-up.

A sublethal temperature treatment, which had produced chlorosis in vivo, reduced the Hill reaction of isolated soybean chloroplasts but not corn chloroplasts.

Lethal temperature treatments reduced the rate of respiration of corn and soybean leaf discs to less than 50 percent of the rate of the control treatment.