92o and 150o; in three of the best folded leaflets of O. fragrans it was 76o, 74o, and 54o.  The angle is often different in the three leaflets of the same leaf.  As the leaflets sink down at night and become folded, their lower surfaces are brought near together (see B), or even into [page 325] close contact; and from this circumstance it might be thought that the object of the folding was the protection of their lower surfaces.  If this had been the case, it would have formed a strongly marked exception to the rule, that when there is any difference in the degree of protection from radiation of the two surfaces of the leaves, it is always the upper surface which is the best protected.  But that the folding of the leaflets, and consequent mutual approximation of their lower surfaces, serves merely to allow them to sink down vertically, may be

Fig. 127.  Oxalis acetosella:  A, leaf seen from vertically above; B, diagram of leaf asleep, also seen from vertically above.

inferred from the fact that when the leaflets do not radiate from the summit of a common petiole, or, again, when there is plenty of room from the sub-petioles not being very short, the leaflets sink down without becoming folded.  This occurs with the leaflets of O. sensitiva, Plumierii, and bupleurifolia.

There is no use in giving a long list of the many species which sleep in the above described manner.  This holds good with species having rather fleshy leaves, like those of O. carnosa, or large leaves like those of O. Ortegesii, or four leaflets like those of O. variabilis.  There are, however, some species which show no signs of sleep, viz., O. pentaphylla, enneaphylla, hirta, and rubella.  We will now describe the nature of the movements in some of the species.

Oxalis acetosella.—­The movement of a leaflet, together with that of the main petiole, are shown in the following diagram (Fig. 128), traced between 11 A.M. on October 4th and 7.45 A.M. on the 5th.  After 5.30 P.M. on the 4th the leaflet sank rapidly, and at 7 P.M. depended vertically. for some time before it assumed this latter position, its movements could, of course, no longer be traced on the vertical glass, and the broken line in the diagram ought to be extended much further [page 326] down in this and all other cases.  By 6.45 A.M. on the following morning it had risen considerably, and continued to rise for the next hour; but, judging from other observations, it would soon have begun to fall again.  Between 11 A.M. and 5.30 P.M. the leaflet moved at least four times up and four times down before the great nocturnal fall commenced; it reached its highest point at noon.  Similar observations were made on two other leaflets, with nearly the same results.  Sachs and Pfeffer have also described briefly* the autonomous movements of the leaves of this plant.

Fig 128.  Oxalis acetosella:  circumnutation and nyctitropic movements of a nearly full-grown leaf, with filament attached to the midrib of one of the leaflets; traced on vertical glass during 20 h. 45m.