Monoecious, and bearing their male flowers in catkins, they are readily distinguished from the rest of the catkin-bearing trees by their peculiar fruit, an acorn or nut, enclosed at the base in a woody cup, formed by the consolidation of numerous involucral bracts developed beneath the fertile flower, simultaneously with a cup-like expansion of the thalamus, to which the bracteal scales are more or less adherent.
These segments spring apparently from the top of the ovary - the real explanation, however, being that the end of the flower-stalk or "thalamus," as it grows, becomes dilated into a sort of cup or tube enclosing and indeed closely adhering to the ovary, so that the latter organ appears to be beneath the perianth instead of above it as in a lily, an appearance which has given origin to the term "inferior ovary."
The termination of the peduncle, or the part on which the whorls of the flower are arranged, is called the thalamus, torus or receptacle.
The axis is usually very much contracted, no internodes being developed, and the portion bearing the floral leaves, termed the thalamus or torus, frequently expands into a conical, flattened or hollowed expansion; at other times, though rarely, the internodes are developed and it is elongated.
These are usually densely crowded upon the thalamus, but in some instances, after apical growth has ceased in the axis, an elongation of portions of the receptacle by intercalary growth occurs, by which changes in the position of the parts may be brought about.
S, the whorl of stamens inserted on the thalamus and surrounding the pistil.
The stamens arise from the thalamus or torus within the petals, with which they generally alternate, forming one or more whorls, which collectively constitute the androecium.
Filaments are usually articulated to the thalamus or torus, and the stamens fall off after fertilization: but in Campanula and some other plants they are continuous with the torus, and the stamens remain persistent, although in a withered state.