Studies on the formation of tuberous root in dahlia. II

Abstract

This observation was conducted to obtain anatomical and morphological aspects of primary root and tuberous root in dahlia. 1. The primary root has a tetrarch protostele, and four groups of small phloem cells alternate with the primary xylem strands, and the pith is in small proportion. 2. The first adventitious root appears on the basal part of the cotyledon, when about 4_??_8 leaves are opened on the stem. Thereafter, as the plant develops, adventitious roots successively appear (until the number reaches 20_??_35) on the cotyledonary node and lower nodes of the stem, these portions are commonly called the crown. 3. These adventitious roots have radial vascular bundles of 12_??_30 strands and pith of relatively large volume. These are the so-called polyarch vascular bundles. In the adventitious roots the secondary xylem is formed by cell division in the cambium and secondary meristem near the vessels. 4. The thickening of adventitious roots is brought about by the cell division mentioned above and by the elongation of these cells, then adventitious roots develop into the tuberous roots. 5. Degree of the thickening of adventitious root is related to the number of rings of secondary vessels and to the area of pith tissue, while the diameter of adventitious roots is not correlated to the number of primary xylems.