APICAL MERISTEM FORMATION DURING ZYGOTIC EMBRYO DEVELOPMENT OF WHITE SPRUCE

The first notable sign of shoot and root meristem development in zygot ic embryos of white spruce (Picea glauca (Moench) Voss) was the appear ance of starch in the respective poles of the embryo. Starch granules gradually accumulated in the subapical cells of the shoot pole and wer e soon followed by vacuolation in the subapical cells. Vacuolation rea ched its highest degree in these cells at the early embryo stage. With the formation of the large vacuolated cells, the surface cell layer a t the summit of the shoot pole enlarged and differentiated into the su rface initials of the shoot meristem. These cells were large with dist inct nuclei. As the embryo matured, the large vacuoles within the suba pical cells were replaced by small ones with a concomitant increase in the cytoplasmic density of the subapical cells. After germination, th e surface initials remained distinct; however, the subapical cells had different fates depending on their location. The subapical cells loca ted next to the surface initials became the central mother cells of th e shoot meristem and those located near the cotyledon junction divided periclinally and gave rise to the epicotyl rib meristem. The remainin g subapical cells near the procambium became part of the pith. In the root pole, starch could be found in the cap region terminating underne ath the developing procambium at the club-shaped stage of embryo devel opment. Two layers of root initials appeared at the junction between t he developing procambium and the root cap. The initials were distinct as they were larger in size than surrounding cells. Cell division acti vity could not be detected in the layer of initials immediately next t o the procambium, while mitotic activity could be seen in the adjoinin g layer next to the root cap. As the embryo matured, cells surrounding the root initials also took on structural characteristics similar to the root meristem initials. As a result, the region of the root merist em initials expanded in size, and the bilayered configuration became o bscure. After germination, mitotic figures could be found in the root initials. As seedling continued to grow, fewer mitotic figures could b e found in the root meristem cells adjacent to the procambium pole. In roots of mature plants, a group of isodiametric cells could be discer ned between the procambium and the root cap. Mitotic activity was not readily detected within this group of isodiametric cells but mitotic f igures could be found in surrounding cells.