THE SHOOT APICAL ONTOGENY OF THE PICEA ABIES SEEDLING. IV. PROTOXYLEM INITIATION AND AGE OF INTERNODES

In Picea abies seedlings the distance below the base of the shoot apical dome to the first protoxylem (px) to be differentiated in the internodes beneath is a linear function of apical dome basal diameter. By using mathematical relations derived in earlier papers of this series, we computed the morphogenic age in plastochrons and the chronometric age in days of the internode in which px is first differentiated (n_px). As the seedlings age from 30 to 140 days, the distance from the base of the apical dome to px increases from 186 to 295 μm, the n of n_px increases from 16 to 53, but the chronometric age of n_px remains within the range of 10 to 12 days. Protoxylem differentiation in young internodes is, therefore, more closely related to chronometrie age than it is to morphogenic age or to distance from the base of the shoot apical dome.