Apical dominance in Xanthium strumarium was influenced by the quality of illumination received at the end of the photoperiod. The involvement of the red/far-red regions of the spectrum was apparent. The persistence of the effects was partially dependent on the age of the individual buds concerned. Plants receiving 30 minutes of illumination from tungsten lamps after a 16-hour photoperiod from fluorescent tubes failed to branch, whereas plants given an identical photoperiod, both in terms of day-length and photosynthetically available light energy, but lacking the far-red from tungsten lamps, branched profusely.The influence of the spectral distribution of illumination on the levels of cytokinins and abseisic acid in the plant, and the correlation with the degree of branching, is presented and discussed. The cytokinin content was much higher in inhibited than released buds. The cytokinins present were probably not able to particinate in bud growth because of an accumulation of inhibitors resembling abscisic acid. The concentration of the inhibitors in inhibited buds was 50 to 250 times that occurring in all other plant parts examined.

SUMMARYA study has been made of the endogenous factors controlling lateral bud initiation and development in the variety Craigella, and two isogenic lines of this variety, Craigella Blind (blbl) and Craigella Lateral Suppressor (isls), in both of which side shoot growth is suppressed to varying degrees. Winter-and summer-sown plants are compared.Lateral Suppressor plants contained higher endogenous levels of auxin in their apices, mature leaves and stems than did normal plants, but lower levels of cytokinins in all parts except the stem, where the levels were similar in both types. The levels of abscisic acid-like substances were consistently higher in the stems of Lateral Suppressor plants than in the corresponding tissue of normal plants, but other plant parts showed a more variable pattern. Lateral Suppressor plants contained higher levels of gibberellin than did normal plants.Blind plants contained higher levels of auxin in the apices and stem than did normal plants, but the levels in mature leaves showed no consistent pattern. Higher levels of abscisic acid-like substances were present in the stem and axillary buds of blind plants than in normal plants, with again a more variable pattern in other plant parts. The levels of cytokinins and gibberellins varied considerably between the two sowing dates.A possible hormonal mechanism of control of axillary bud development is discussed.

SUMMARY A greatly improved method is described for the bio‐assay of abscisic acid (ABA) and other compounds that possess ‘antitranspirant’ activity. As in the previous method, the stomatal responses are observed on pieces of isolated epidermis of Commelina communis immersed in small volumes of solution containing the compounds to be assayed. In new media, it is now possible to obtain linear responses to ABA concentrations over the range 10‐4‐10‐8 M in PIPES buffer at pH 6.8, and over the range 10‐7‐10‐10 M in citrate buffer at pH 5.5. In citrate buffer, it is possible to detect as little as 26 pg of ABA. In both media, the assay is unaffected by the presence of six other growth regulators (auxin, gibberellic acid, kinetin, coumarin, xanthinin and scopoletin) but the stomata closed partially in response to 10‐3 M chlorogenic acid.