For our investigation two species of herbaceous plants were selected with different morphological and anatomical stem structures: Japanese knotweed and topinambur. The distribution of chlorophyll (chl) in photosynthetically active tissues, divided into younger and older parts of stems, was studied by the use of the chl autofluorescence phenomenon. The content of chl in the stems of the topinambur was lower than in leaves by ca. 40-50%. In the J. knotweed, the quantity of chl was lower by ca. 70% in the young fragments of stems than in leaves, whereas in the older parts it approached the levels found in leaves. The chl a/b ratio was generally higher in leaves compared with stems. It was also found that the maximal efficiency of PSII (F_v/F_m) did not differ greatly between leaves and stems in these two plant species. Despite this, the maximal net photosynthetic rates (P_N) in the stems of both species were low and kept to level of 0-0.5 μmol·m^-2·s^-1. Much higher levels of PN were noted in the leaves of J. knotweed at ca. 10 and ca. 14 μmol·m^-2·s^-1 in the topinambur. The stems of herbaceous plants are characterized by high resistance of the epidermis, although they do not have a cork limiting light and transpiration. As a result, similar to lignified, the stems of herbaceus plants use mainly internal CO₂ from respiration in the process of photosynthesis.