Many, if not most, trees contain greenish, photosynthetically active chlorenchyma tissue below the outer periderm or rhytidome of branches and even stems. However, not so many people have realized their ecological significance and photosynthetic function. Here, by reviewing previous studies and our own research work, some general conclusions were got as following:
（1）Branch（stem）photosynthesis (difference between dark respiration rate and respiration at saturation light regime) was 0~10μmol m^-2s^-1, while most of the data were ranged from 0.5 μmol m^-2s^-1 to 3.0 μmol m^-2s^-1. Although net photosynthetic uptake of CO₂ is rarely found, stem internal re-fixation of CO₂ in young branches and stems may compensate for the potential respiratory carbon loss. Over 80% of the referenced data shows that this photosynthetic uptake could refix 40%~100% of the respiratory CO₂ loss. However, a few studies have scaled up from instantaneous studies to tree or forest level.
（2) Not only the photosynthetic carbon-fixation itself, but also photosynthates from canopy leaves strongly affect the respiratory CO₂ loss from non-photosynthetic organs. However, there are still discrepant opinions on the functional difference of newly-formed photosynthates and stored photosynthates.
（3) Chlorophyll content in barks is generally 80~450 mg m^-2，which is lower than those in corresponding leaves (340-620mg m^-2). Moreover, the ratio between chlorophyll a and chlorophyll b, chl. a/b for barks is 2.5 in average, which is significantly lower than that of leaves, 3.6 in average (p<0.0001), indicating shade-environmental acclimation of barks.
（4) Traditional view on the function of carotenoids for non-photosynthetic organs is its low light acclimation since the total content at bases of area and dry mass in barks are much lower than that in leaves. However, recent studies have shown that total carotenoids and its componential compounds at a base of chlorophyll fresh mass are much higher than that corresponding leaves. This has proved to be related to the acidified chloroplast stroma by extremely high CO₂ concentration in barks. It is suggested that high internal CO₂ concentrations in barks impede photosynthesis possibly through acidification of protoplasm and impairment of the pH-dependent high energy state quenching followed by reduction in the efficiency of heat dissipation.
（5) Optics for the barks of stems and branches are characterized as that most of light being absorbed and only part of light being reflected and transmitted. About 70% of the data show that transmission is ranged from 5% to 15%, while over 85% show that the transmission is ranged from 0% to 20%. Moreover, vessels, fibres (both xylem and phloem fibres) and tracheids in woody plants can also conduct light efficiently along the axial direction, via their lumina (vessels) or cell walls (fibres and tracheids). Only the spectral region of far-red and near infra-red light is efficiently conducted and transmitted in branches and stems.
（6) Although some study on herbal plants showing a C4 feature, many other species, particularly woody branches and stems is unknown. The influences of special microenvironment (extremely high CO₂ concentration, increased red to blue photon ratio, and hypoxia) on the photosynthetic machinery need further studies.