We have previously hypothesized that the cause of figured wood formation in Karelian birch (Betula pendula Roth var. carelica) is excessive sucrose supply to the phloem and the cambial zone. The ability of cells to sense the flux of sugars rather than sensing the presence of sugars in the intra- and extracellular space forms the basis of an efficient sugar sensing machinery. Therefore, transport sucrose plays a special role in terms of impact on the morphogenesis of plant cells and tissues. Sucrose flow affects the sensors that trigger signals transmitted to sugar-modulated genes. We investigated the pool of sucrose in trunk tissues of common silver birch (B. pendula var. pendula) and Karelian birch taking into account transport and storage forms of the disaccharide. For this purpose the sucrose content and the activity of enzymes which break down it (sucrose synthase, invertases: apoplastic, vacuolar, cytoplasmic) were determined. It was found that during the period of cambial growth, sucrose is almost the only sugar in the phloem exudate of Karelian birch as in common silver birch, i.e. it serves as the main transport form of carbohydrates. Data on sugar content and the activity of enzymes were compared to show that: (1) sucrose in birch phloem does not perform the storage function, (2) the entire sucrose pool here should be regarded as transport sucrose, (3) during the cambial activity period the role of the labile sugar pool in birch phloem is performed by fructose, (4) fructose in birch is the storage sugar during winter dormancy, as well as a cryoprotectant.