THE FORMATION OF NODAL ADVENTITIOUS ROOTS IN SALIX CORDATA

Abstract

IT IS well known that stem-cuttings of certain willows, poplars, and other plants root very readily when placed under favorable conditions and that the adventitious roots usually appear at definite places on the stems; but no detailed investigation of the time and method of development of such roots has been published. Trecul (1846) studied the position of adventitious roots on cuttings of many species and suggested the possibility of the existence in certain species of primordia of roots in the stems which remain latent until the cuttings of these plants are placed under conditions favorable for the growth of roots. Vochting (1878) noted the regular arrangement of roots on cuttings of willow and other plants and found the primordia of roots in twigs three to four months old. He thought that the primordia must be present in still younger twigs, but too small to be seen. Van der Lek (1924) made a more detailed study of rootformation in cuttings of several species of Salix and Populus, in Ribes nigrum, and in Vitis vinifera and found root primordia in cuttings made in January, February, and March. In Ribes nigrum, primordia occur in three double series beginning immediately under the insertion of each leaf and extending downward through the internode; a few appear in one or more vertical series on the opposite side of the internode. The primordia lie outside the cambium. Those in series directly beneath the leaf are formed in those rays limiting the wood sectors which become the traces of the leaf in question; those on the opposite side are similarly related to the leaf next above. The rays are locally widened at the places where the root primordia appear. Each primordium is formed immediately under a lenticel. In Salix, Van der Lek found both nodal and internodal root primordia, the nodal ones closely connected with the wide radial bands of parenchyma formed by the leaf strands leaving the wood cylinder. The internodal ones are similar in location to those of Ribes nigrum. In Populus, the root primordia are formed opposite the centers of the sectors which become the traces to the leaf above, rather than opposite the rays which bound these sectors. No pre-formed root primordia were found in Salix caprea, S. aurita, Populus alba, or Vitis vinifera. The present study was made to determine the time of origin and the method of development of the latent root primordia in nodes of stems of Salix cordata Muhl. Beginning early in May, when the buds had just begun to grow, shoots of this willow were collected at intervals until early autumn. Nodal regfions of shoots were prepared for sectioning in the