Fig. 141.  Trifolium repens:  A, leaf during the day; B, leaf asleep at night.

ing and bends over until it rests on and forms a roof over the edges of the now vertical and united lateral leaflets.  Thus the terminal leaflet always passes through an angle of at least 90o, generally of 130o or 140o, and not rarely—­as was often observed with T. subterraneum—­of 180o.  In this latter case the terminal leaflet stands at night horizontally (as in Fig. 141), with its lower surface fully exposed to the zenith.  Besides the difference in the angles, at which the terminal leaflets stand at night in the individuals of the same species, the degree to which the lateral leaflets approach each other often likewise differs.

We have seen that the cotyledons of some species and not of others rise up vertically at night.  The first true leaf is generally unifoliate and orbicular; it always rises, and either stands vertically at night or more commonly bends a little over so as to expose the lower surface obliquely to the zenith, in the same manner as does the terminal leaflet of the mature leaf.  But it does not twist itself like the corresponding first simple leaf of Melilotus. [page 350] With T. Pannonicum the first true leaf was generally unifoliate, but sometimes trifoliate, or again partially lobed and in an intermediate condition.

Circumnutation.—­Sachs described in 1863* the spontaneous up and down movements of the leaflets of T. incarnatum, when kept in darkness.  Pfeffer made many observations on the similar movements in T. pratense.** He states that the terminal leaflet of this species, observed at different times, passed through angles of from 30o to 120o in the course of from 1 ½ to 4 h.  We observed the movements of T. subterraneum, resupinatum, and repens.

Trifolium subterraneum.—­A petiole was secured close to the base of the three leaflets, and the movement of the terminal leaflet was traced during 26 ½ h., as shown in the figure on the next page.

Between 6.45 A.M. and 6 P.M. the apex moved 3 times up and 3 times down, completing 3 ellipses in 11 h. 15 m.  The ascending and descending lines stand nearer to one another than is usual with most plants, yet there was some lateral motion.  At 6 P.M. the great nocturnal rise commenced, and on the next morning the sinking of the leaflet was continued until 8.30 A.M., after which hour it circumnutated in the manner just described.  In the figure the great nocturnal rise and the morning fall are greatly abbreviated, from the want of space, and are merely represented by a short curved line.  The leaflet stood horizontally when at a point a little beneath the middle of the diagram; so that during the daytime it oscillated almost equally above and beneath a horizontal position.  At 8.30 A.M. it stood 48o beneath the horizon, and by 11.30 A.M. it had risen 50o above the horizon; so that it passed through 98o in 3 h.  By the aid of the tracing we ascertained that the distance travelled