Some characteristics of uptake of 18-14CIN"-benzyladenine (BA) by watermelon (Citdus vulgaris Schrad., cv. Fairfax) cotyledons that were either excised immediately after 24 hours inhibition (day 0) or cultured in the dark for 48 hours on moist filter paper (day 2) have been compared.The uptake of BA seems to be passive in cotyledons of both kinds. The initial rate of uptake is, however, much slower In day 2 cotyledons. This is probably due to a higher resistance of cell membranes to BA influx. When the day 2 cotyledons are frozen and thawed, so that the membrane barrier is abolished, the amount of BA taken up is the same as in day 0 cotyledons.In spite of the lower rate of uptake, the physiological effects of BA in day 2 cotyledons are as strong as in day 0 cotyledons and occur with a shorter lag time. Sensitivity to BA seems indeed to start 24 to 48 hours after excision.Exogenous cytokinins accelerate development of excised cotyledons (2,4,7,15). We have shown (6) that continuous exposure to BA is not required for maximal effect. A treatment of 1 to 4 h with 10-' M BA is optimal, whereas a treatment of a few minutes suffices for a significant response. We used, for these experiments, cotyledons excised from the embryo after 24 h of imbibition (day 0). In these cotyledons, the responses to BA become apparent after 24 to 48 h lag time. In the present study, we observed that the lag can be greatly shortened if BA is supplied to cotyledons that have been grown in water for 2 d after excision.In a former paper (3), we described the main characteristics of BA uptake in cotyledons on day 0. The much prompter response of day 2 cotyledons suggested that BA uptake could be more efficient at this stage of development. In the present paper, we compare the characteristics of uptake by the two kinds of cotyledons.MATERIALS AND METHODS Watermelon (Citrullus vulgaris Schrad., cv. Fairfax) seeds were bought from SRS Seeds, Modesto, CA. The cotyledons were excised from the embryonic axis after 24 h of imbibition in distilled H20 at 27°C (day 0). They were then grown for 48 h more on moist filter paper in Petri dishes at 27°C in the dark.These day 2 cotyledons were used for the uptake tests.