This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
This neat member of a charming family is by no means a common plant; in fact, northward of South Wales and Norfolk it is unknown. Southward it may be found in hedges and waste places, flowering in June and July. The stems are slight, and greatly swollen at the joints. The leaf-stalks are long, and the leaves, though their general outline is kidney-shaped, are deeply cut into about seven lobes, which are in turn lobed or toothed. Owing to the close general resemblance of this species to its immediate congeners some rather minute differences should be noted. The sepals end each in a hard point - in botanists' language they are mucronate - the margin of the narrow petals is entire, that is, not notched, and the narrow lower portion (claw) is not fringed with hairs. The carpels, or divisions of the seed-vessel, are keeled but not wrinkled, and the seeds are pitted. Its nearest allies are:
I. The Dove's-foot Crane's-bill (G. molle), with similar leaves to the last, but with notched petals, the claw bearded. Flowers more rosy than rotundifolium.
II. Small-flowered Crane's-bill (G. pusillum). Leaves more deeply lobed, sepals as long as the notched petals, claw slightly hairy. Flowers, pale rose.
III. Long-stalked Crane's-bill (G. columbinum). Lobes of leaves distant from each other, the segments into which they are again cut being very narrow ; sepals large, acuminate and awned, as long as the entire rose-purple petals; claws less hairy than in last. All the leaf and flower-stalks long.
IV. Cut-leaved Crane's-bill (G. dissection). Similar to G. columbinum, but all stalks much shorter. Bright red petals, notched.
V. Herb-Robert (G. robertianum). Plant more or less red. Leaves divided into five leaflets, these again divided. Calyx angular, the sepals long-awned and hairy. Petals narrow and entire; purple streaked with red; claw smooth.
VI. Shining Crane's-bill (G. lucidum). Plant more or less crimson in summer. Leaves divided into five segments, each bluntly lobed at the top. The calyx is a wrinkled pyramid, each sepal awned. The rosy petals are much longer than the sepals; claw smooth. There are two lines of hairs on the upper branches.
All the above are annual or biennial plants. The name of the genus is from the Greek geranos, a crane, from a fancied resemblance in the fruit to a Crane's-bill.
The mechanism for the dispersal of seeds in the Crane's-bills is worthy of attention. When the petals fall off the carpels enlarge, and the outer layer of the style separates from the axis, splitting into five portions, each attached to a carpel at the bottom and to the style at top. The axis of the style further elongates, but the tails of the carpels do not, and there is, in consequence, great tension, which ends in the carpel being detached from its base. The "tail" curls up, the carpel is reversed, and the seed drops out.