[It has long been known* that when the sun shines brightly on the leaflets of Robinia, they rise up and present their edges to the light; whilst their position at night is vertically downwards.  We have observed the same movement, when the sun shone brightly on the leaflets of an Australian Acacia.  Those of Amphicarpaea monoica turned their edges to the sun; and an analogous movement of the little almost rudimentary basal leaflets of Mimosa albida was on one occasion so rapid that it could be distinctly seen through a lens. the elongated, unifoliate, first leaves of Phaseolus Roxburghii stood at 7 A.M. at 20o above the horizon, and no doubt they afterwards sank a little lower.  At noon, after having been exposed for about 2 h. to

* Pfeffer gives the names and dates of several ancient writers in his ’Die Periodischen Bewegungen,’ 1875, p. 62. [page 446]

a bright sun, they stood at 56o above the horizon; they were then protected from the rays of the sun, but were left well illuminated from above, and after 30 m. they had fallen 40o, for they now stood at only 16o above the horizon.  Some young plants of Phaseolus Hernandesii had been exposed to the same bright sunlight, and their broad, unifoliate, first leaves now stood up almost or quite vertically, as did many of the leaflets on the trifoliate secondary leaves; but some of the leaflets had twisted round on their own axes by as much as 90o without rising, so as to present their edges to the sun.  The leaflets on the same leaf sometimes behaved in these two different manners, but always with the result of being less intensely illuminated.  These plants were then protected from the sun, and were looked at after 1 ½ h.; and now all the leaves and leaflets had reassumed their ordinary sub-horizontal positions.  The copper-coloured cotyledons of some seedlings of Cassia mimosoides were horizontal in the morning, but after the sun had shone on them, each had risen 45 1/2o above the horizon. the movement in these several cases must not be confounded with the sudden closing of the leaflets of Mimosa pudica, which may sometimes be noticed when a plant which has been kept in an obscure place is suddenly exposed to the sun; for in this case the light seems to act, as if it were a touch.

From Prof.  Wiesner’s interesting observations, it is probable that the above movements have been acquired for a special purpose. the chlorophyll in leaves is often injured by too intense a light, and Prof.  Wiesner* believes that it is protected by the most diversified means, such as the presence of hairs, colouring matter, etc., and amongst other means by the leaves presenting their edges to the sun, so that the blades then receive much less light.  He experimented on the young leaflets of Robinia, by fixing them in such a position that they could not escape being intensely illuminated, whilst others were allowed to place themselves obliquely; and the former began to suffer from the light in the course of two days.