The Developmental Basis of Anisophylly in Selaginella martensii. II. Histogenesis

The anisophyllous shoots of Selaginella martensii are characterized by opposite pairs of dorsal and ventral leaves which differ in size and form. This paper documents the processes of histogenesis for each leaf type and shows that both leaf types show a strong basipetal gradient of tissue maturation as indicated by cell size, distribution of cell division, cell vacuolation, intercellular space formation and stomatal differentiation. These processes are complete at an earlier stage of development in dorsal leaves (P7–8) than for ventral leaves (P12, 13). Measurement of cell number in the uppermost dermal layer indicate that cell divisions occur at a greater rate during ventral leaf development and observations of mitotic figures indicate that cell division occurs over a slightly longer time period in ventral leaves (through P6) as compared with dorsal leaves (through P5). While the morphological and histological differences that occur between the mature ventral and dorsal leaves begin to appear at or shortly after leaf inception, the general pattern of histogenesis of both types of leaf is similar. Histologically the dorsal leaf is primarily distinguished by a precocious maturation of tissues.