WATER RELATIONS IN BRYOPHYLLUM CALYCINUM SUBJECTED TO SEVERE DRYING

Bryophyltum calycinum Salisb. will reproduce vegetatively by means of plantlets formed in the notches of the vegetative leaves. This reproduction, called regeneration by LOEB (9), has been rather thoroughly treated by LOEB (10), REED (17), BRAUN (1), CHILD and BELLAMY (2), and others. A great deal of attention has been paid by these authors to the reproduction of the young plants, but little attention has been given the water relations in the parent leaf during their formation and-growth. In most of the work reported the leaves, when removed from the plant, were placed in contact with water or at least in a moist atmosphere. Under laboratory conditions, without extraneous water, the plantlets appear on the margin of all portions of the leaf without regard to polarity (fig. 1, A, B). The leaves, when hung up by their petioles, produce young plants on the basal half as well as on the apical half, but when placed with one edge in water or in moist soil the shoots will appear only where the leaf is in contact with the water or moist soil. The parent leaf may dry until it becomes brittle at its central portion, yet the young plants along the margin will still be turgid and continue to increase in size. Since this leaf is capable of living for a long period and reproducing new tissues without additional water being supplied to it, it was proposed that a study be made of the water relations of the parent leaves and their plantlets. A great number of phenomena might be studied in connection with the ability of these leaves to withstand severe drying: osmotic concentration of the juices of the leaves and plantlets, electrical conductivity of these juices, hydration of the ions present in the solutions, relationship of the freezable and unfreezable water to the total water, the thickness of the cuticle, the distribution of stomata, and so on. The work presented in this paper will deal with the freezable and unfreezable water in their relation to the total water, the thickness of the cuticle, and the distribution of stomata. Unfreezable water has attracted a great deal of attention among plant physiologists who were interested in cold resistance or the ability of plants to become hardened against killing at low temperatures. ROSA (19) found that in cabbage the rate of decrease in the percentage of freezable water during hardening coincided with the rate of hardening of plants against killing by low temperatures. GREATHOUSE and STUART (6) found that