The brown alga C. barbata from the Black Sea was studied and a quantitative description was given of the relationship between the total length (L), weight (W) and age (T) of its thallus as well as between W, T and the intensity of 2 physiological functions, photosynthesis (Pi) and the consumption of glucose dissolved in the water (PO). The relationship between the length (l), weight (w) and age (t) of the primary branches of Cystoseira was also studied, as well as between w, t and Pi and PO in the branches. The difference between 2 local Cystoseira populations, undistinguishable by the correlation T-L (a less variable character), is clearly revealed on the basis of the T-W relation (a more variable character). A physiological age periodicity of the ontogenesis of the thallus (early youth, maturity, aging and senescence) is given, based on the T-W and T-L relations. The ontogenesis of the branch repeats the ontogenesis of the thallus with other quantitative characteristics: in the development of the thallus W and L change non-synchronously, while in the development of the branch w and l change synchronously with each other. The physiological pattern in the age series of primary branches is an increase in Pi and PO up to maximum, at the age of months, followed by a considerable decrease of their intensity. The ecological significance of the observed relations and their consequences are discussed in regard to the experimental determination of the production of Cystoseira and the assessment of the intensity of its chemical interaction with the marine environment.