THE DEVELOPMENTAL BASIS OF ANISOPHYLLY IN SELAGINELLA MARTENSII. I. INITIATION AND MORPHOLOGY OF GROWTH

The dorsiventral shoot system of Selaginella martensii is characterized by opposite pairs of ventral and dorsal leaves that are dimorphic in size and form. This study was undertaken to determine if the smaller dorsal leaf can be appropriately regarded as an arrested form of the larger leaf. Although the pattern of cell divisions and cell enlargement associated with leaf initiation is similar for both leaf types, the extent of localized growth results in distinctly larger primordia on the ventral side of the shoot. Ventral leaf primordia are also distinguished by the early formation of more extensive mesophyll tissue. Regression analysis of quantitative data on leaf length vs. position and leaf width vs. length indicates that the growth pattern of ventral and dorsal leaves is significantly different. These observations indicate that the developmental pathways of the dimorphic leaves of Selaginella martensii do not diverge at a relatively late developmental stage, but rather can be distinguished from inception.