Function of transfer cells in the nodal regions of stems,particularly in relation to the nutrition of young seedlings

Summary The distribution and time course of development of transfer cells in the hypocotyl region of lettuce (Lactuca sativa L.) and groundsel (Senecio vulgaris L.) are examined by light microscopy of serial sections through a sequence of ages of hypocotyls. Investments of xylem transfer cells occur in departing traces to the cotyledons and, later, in the traces to foliage leaves; phloem transfer cells are widely distributed but particularly prominent in those bands of protophloem in the plumule vasculature which lie alongside xylem of the cotyledonary traces. Both classes of transfer cell are well endowed with wall ingrowths before differentiation of xylem and perforation of stomata occurs in the plumule. Autoradiographic evidence is obtained of a transport pathway from cotyledonary trace xylem elements to xylem transfer cell to plumule, and analyses of xylem sap collected from above or below the zones of transfer cells in the hypocotyl show that certain materials can be removed from the xylem sap by transfer cells as it moves towards the cotyledons. From these findings it is concluded that the seedling transfer cells play an important role in nutrition of the young plumule, particularly before the latter has become adequately connected with the vascular systems of cotyledons and root.Experiments on the experimental modification of transfer cell development in the hypocotyl suggest that both photosynthetic fixation of carbon dioxide and a transpirational loss of water by a cotyledon must take place before the presumptive xylem transfer cells in its traces can develop normal sets of wall ingrowths.Discussion is extended to the general role of transfer cells in the nodal regions of stems. Possible functions envisaged are, the general nutrition of young tissues of the apical region, the abstraction of assimilates for local storage, the transfer of assimilates to axillary buds released from apical dominance, and the interchange of assimilates between adjacent vascular traces running through the node.