638 LECTURE XXXVI. 



these heliotropic curvatures and those which bring about the sleep-movements, hes 

 in the fact that the organs can make heUotropic curvatures in all directions, e.g. 

 left and right, so that the one flank of the motile organ becomes convex and the 

 other concave, according to which side the light falls on. The movement of waking 

 and sleeping, on the contrary, only takes place as already mentioned in one plane, 

 which divides the leaf and the motile organ symmetrically; and it is thus unimportant 

 here in what direcdon the rays of light fall upon the motile organ, but only important 

 that Hght is present at all and increases or decreases in intensity. 



The above will suffice for the distinction of the movements of waking and 

 sleeping from the heliotropic curvatures. In order to obtain a shorter expression 

 for the former, however, I have since 1865 proposed to term those effects of light 

 which cause the opening of the leaves as the intensity increases, and their closure as 

 darkness comes on, Paraionic effects ; because they only take place if the leaves are 

 in the normal vital condition which I term Phoiotonus. If such leaves are endrely 

 excluded from the light for several days, they pass into a condition of rigidity due to 

 the darkness — i.e. they are no longer capable of being set in movement by variations in 

 the light, though this recurs after long exposure to light, by which means the phototonus 

 is restored. We may thus say shortly, the movements of waking and sleeping are 

 called forth by paratonic light-stimuli, whereas the spontaneous movements of the 

 same leaves are independent of any light-stimuli, but probably depend on phototonus ; 

 heliotropic curvatures, on the contrary, have nothing to do with phototonus. 



If we now wish finally to distinguish the daily periodic movements induced by para- 

 tonic light-stimulus, from the various other movements of the same leaves, often combined 

 with them, I must, in conclusion, notice a phenomenon which is perhaps the most 

 confusing one of all in this province. I have so far assumed that the waking leaves, in 

 the diurnal position, only receive ordinary bright daylight, or if the direct rays of the 

 sun only temporarily and not too strong. However, if they are exposed to very 

 intense insolation, especially about noon, the leaves close up and assume what looks 

 like the nocturnal position. We have here, in fact, again to do with the profile position 

 characterised by Stahl, which I have already described in detail in the previous 

 lecture as regards the chlorophyll-grains, and have referred to as regards the leaves. 

 Not only leaves which are paratonic and irritable, but others also have this pecu- 

 liarity, that under very strong illumination they undergo curvatures or torsions, by 

 means of which the lamina is so placed that the fierce rays of the sun are parallel to 

 it, or at any rate fall upon it at a very acute angle ; by this means a too intense action 

 of the light is avoided. 



It will readily be admitted that it is no easy task to detect the differences 

 between all these various influences which cause alterations in the positions and dis- 

 position of the leaves, and to keep them apart and refer each to its causes, and 

 eventually to recognise in its undisguised form the true daily periodicity which may be 

 combined with all these variations. I first succeeded in 1863 in separating the spon- 

 taneous periodic movements from the paratonic ones, and in distinguishing between 

 phototonus and rigidity due to darkness; and Pfeffer in 1875 first recognised the true 

 daily periodicity as a phenomenon made up of a direct paratonic action and its after- 

 effect, previous observers having already established the fact that the leaves endowed 

 with daily periodicity are sensitive to mere darkening and illumination. 



