Structural Changes in the Vascular Cambium of Pinus strobus L. during an Annual Cycle

Abstract

The changes in the ultrastructure of cambial cells of eastern white pine (Pinus strobus L.) during an annual cycle are observed and recorded as are relationships of cambial cells during dormancy and at resumption of cambial activity. Cambial activity was resumed late in March or early in April, when a few cells divided periclinally. Cambial activity reached a maximum during the latter part of May with 15 to 20 undifferentiated cells present. In July it declined markedly, and the number of undifferentiated cells equalled that of the dormant period. The xylem and phloem tissue cells produced late in the annual cycle overwintered at varying developmental stages. In October cambial cells structurally resembled dormant cells. The number of dormant cells in eastern white pine cambium varied from 6 to 10. Active cells were characterized by a large central vacuole, by an abundance of all cell organelles, and by thin cell walls. Dormant cells were characterized by numerous small vacuoles, by structurally and quantitatively modified cell organelles, and by relatively thick cell walls.