Theoretically the plant is a leaf. The axis or stem is a fused series of midribs. The floral organs are leaves variously modified. Nay, we may look even beyond the leaf to the components of the leaf, for thought cannot rest till it finds the ultimate unit. This is a cell, and the lowest plant is simply a cell. Cells adherent to cells form a frond. A frond, by a little modification, passes into a leaf, and a leaf into a part of the flower. A sepal is a leaf changed but little; a petal is a leaf changed in color and texture; a stamen is a leaf changed in color, texture, and form, the blade being eliminated except at the tip where it forms the anther, and the midrib remaining as a supporting filament; the pistil is a leaf with the lower part of the blade rolled up, and the edges united to form a carpel, and the midrib prolonged into a style, bearing atop a little shred of altered leaf-blade called a stigma. And here—at the very tips of these inner leaves where the nutriment is least and the vital force weakest—the investing membrane disappears; Nature slips back toward her simplest types, and shows us the naked primordial cells as pollen-grains! In the thistle one may see that the stem is continuous with the midrib of the leaf. Every observer knows that in the pond-lily (Fig. 8) we see the intermediate stages between a simple leaf and a pistil. And every one whose garden furnishes a syringa or double-flowering cherry, has only to look at the flowers to see pistils and stamens reverting to leaves.
a a, carpels; b, remnant of pistils; c c, remnants of corolla; d d, remnants of calyx; f, fibrous line.
Fortunately for science, there grows in Kittanning, Pennsylvania, an apple-tree, which, in its flower and fruit, exemplifies the theory of the plant. Theoretically, a fruit is a branch with its leaves transformed.