A CORRELATION BETWEEN THE APICAL CELL AND THE HETEROBLASTIC LEAF SERIES IN MARSILEA

Abstract

Plants of three species of Marsilea (M. vestita, M. villosa, M. drummondii) were grown in sterile culture under controlled conditions, and stem apices were sampled at one of the three heteroblastic leaf forms typical of this plant: spatulate, bifid, or quadrifid leaves. Statistical analyses were made of the relationship between the area of the apical cell and the leaf form which the plant produces under varied growth conditions. For all three species there is a statistically significant correlation (1% level) between apical cell area and leaf form. The analysis indicates that 83% of the variation in apical cell area in Marsilea vestita, 52% in M. villosa, and 54% in M. drummondii can be related to the change in leaf form. An increase in the glucose concentration of the culture medium increases the average apical cell area, and the addition of the protein synthesis inhibitor 2‐thiouracil at concentrations of 10 mg/liter and 25 mg/liter decreases the average apical cell area. A certain average apical cell area is necessary for the production of a particular leaf form in the series. The area is a relative rather than an absolute size under any one particular growth condition. Any growth condition which inhibits or reverses the increase in apical cell size (which is typical of the normal growth pattern) will inhibit or reverse the normal heteroblastic series. Under constant conditions, the average size at which particular leaf forms develop appears to be species specific. These results confirm a generally held idea that the apex size is related to the heteroblastic leaf series, and they indicate that the area of the apical cell is the key factor rather than the volume or the number of cells of the apex. Undoubtedly the area of the apical cell is only a reflection of the physiological or morphological characteristics of the apical meristem that underlie the heteroblastic leaf series and which currently do not appear to lend themselves to more direct quantitative analysis.