This section is from the book "Nature's Garden", by Neltje Blanchan. Also available from Amazon: Nature's Garden; An Aid To Knowledge Of Our Wild Flowers And Their Insect Visitors.
Flowers - Yellow, regular, 5-parted, about 1/4 in. across; 2 or 3 together in the axils. Stem: Weak, 6 to 15 in. tall, branching, leafy. Leaves: Alternate, sensitive, compounded of 12 to 44 small, narrowly oblong leaflets; a cup-shaped gland below lowest pair; stipules persistent. Fruit: A pod, an inch long or more, containing numerous seeds.
Preferred Habitat - Dry fields, sandy wasteland, roadsides.
Flowering Season - July - October.
Distribution - New England westward to Indiana, south to Georgia and Texas.
How many of us ever pause to test the sensitiveness of this exquisite foliage that borders the roadsides, and in appearance is almost identical with the South American sensitive plant's, so commonly cultivated in hothouses here? Failing to see its fine little leaflets fold together instantly when brushed with the hand, as they do in the tropical species (Mimosa pudica), many pass on, concluding its title a misnomer. By simply touching the leaves, however roughly, only a tardy and slight movement follows. A sharp blow produces quicker effect, while if the whole plant be shaken by forcibly snapping the stem with the finger, all the leaves will be strongly affected; their sensitiveness being apparently more aroused by vibration through jarring than by contact with foreign bodies. The leaves, which ordinarily spread out flat, partly close in bright sunshine and "go to sleep" at night, not to expose their sensitive upper surfaces to fierce heat in the first case, and to cold by radiation in the second. "Lifeless things may be moved or acted on," says Asa Gray; "living beings move and act - plants less conspicuously, but no less really than animals. In sharing the mysterious gift of life they share some of its simpler powers."
The Partridge Pea or Large-flowered Sensitive Plant (C. Chamaecrista) likewise goes to sleep; the ten to fifteen pairs of leaflets which, with a terminal one, make up each pinnate leaf, slowly turning their outer edges uppermost after sunset, and overlapping as they flatten themselves against their common stem until the entire aspect of the plant is changed. By day the expanded foliage is feathery, fine, acacia-like; at night the bushy, branching, spreading plant, that measures only a foot or two high, appears to produce nothing but pods. These leaves respond slowly to vibration, just as the sensitive pea's do. In spite of their names, neither produces the butterfly-shaped (papilionaceous) blossom of true peas. The partridge pea bears from two to four showy flowers together, each measuring an inch or more across, on a slender pedicel from the axils. It fully expands only four of its five bright yellow petals; they are somewhat unequal in size, the upper ones, with touches of red at the base, as pathfinders, not, however, as nectar-guides, since no sweets are secreted here. Curiously enough, both right and left hand flowers are found upon the same plant; that is to say, the sickle-shaped pistil turns either to the right or the left. One lateral petal, instead of being flexible and spread like the rest, stands so stiffly erect and incurved that it commonly breaks on being bent back. Why? The pistil, it will be noticed, points away from the ten long black anthers. Obviously, then, the flower cannot fertilize itself. Its benefactors are bumblebee females and workers out after pollen. Cup-shaped nectaries ("extra nuptial") are situated on the upper side and near the base of the leaf stalks on these cassia plants, where they can have no direct influence on the fertilization of the blossoms. Apparently, they are free lunch-counters, kept open out of pure charity. Landing upon the long black anthers with pores in their tips to let out the pollen, the bumblebees "seize them between their mandibles," says Professor Robertson, "and stroke them downward with a sort of milking motion. The pollen . . . falls either directly upon the bee or upon the erect lateral petal which is pressed close against the bee's side. In this way the side of the bee which is next to the incurved petal receives the most pollen. ... A bee visiting a left-hand flower receives pollen upon the right side, and then flying to a right-hand flower, strikes the same side against the stigma." When we find circular holes in these petals we may know the leaf-cutter or upholsterer bee (Megachile brevis) has been at work collecting roofs for her nurseries (see page 61). The partridge pea, which has a more westerly range than the sensitive pea's, extends it southward even to Bolivia. Game birds, migrants and rovers, which feed upon the seeds, have of course helped in their wider distribution. The plant blooms from July to September.