The Structure and Function of the Border Parenchyma and Vein-Ribs of Certain Dicotylendon Leaves

The border parenchyma has long been recognized as a structural constant in the many kinds of angiosperm leaves. As an unbroken sheath it invests all minor veins and is an intermediary between the conductive channels and the mesophyll. Though the presence of the border parenchyma has long been known, references to it have been casual and its significance has not been adequately appreciated. The uniform presence of this sheath in an organ marked by economy of tissue has invited this attempt to evaluate its place in the foliage leaf as a whole. Schubert (24) discussed the structure of the border parenchyma in various plant families and recognized two classes of starch sheaths : those in which there was nerve parenchyma associated with the sheath, and others in which it was lacking. The former refers undoubtedly to the vein-ribs described later in the present paper while the latter is the border parenchyma proper. The nerve parenchyma , he said, was generally present in large leaves and absent in small ones. This generalization was found to be incorrect for the sixty species studied in this research. Schubert was more accurate in his suggestion that it is possible that the nerve tissue assists in the transport of assimilate