640 LECTURE XXXVI. 



petioles and leaflets lay themselves together, the lever on which their centre of 

 gravity acts becomes longer, and thereby the moment of rotation of the leaf- 

 stalk so increased, that it sinks in the evening, and its motile organ curves 

 downwards, although it strives, in consequence of the action of the light in itself, 

 to erect itself as in the case of the Bean. I give these remarks on Pfeffer's 

 authority, not having yet had an opportunity of making decisive observations for 

 myself. 



There are perhaps not many opportunities where it is possible to make so 

 clear to the non-physiologist the extraordinary difficulties which are often con- 

 nected with the investigation of the phenomena of life as here, in the case of 

 the sleep-movements of compound leaves, and it is chiefly on this account that 

 I have dwelt on them at so great a length. 



As already stated, there are also numerous foliage-leaves, at the base of 

 which no specially developed motile organ exists, and where also the indi- 

 vidual parts of the leaf are not sharply separated, and not connected with the 

 stalk or mid-rib by means of special motile organs, but in which, nevertheless, 

 very evident or slightly perceptible sleep-movements as well of the whole leaf 

 as also of its parts take place, and this again in such a way that the entire 

 leaf rises and falls, approaching or retiring from the upper part of the shoot- 

 axis, while in many cases the lateral ribs of the lamina curve at the same time 

 upwards or downwards, leaving the lamina flat or curved. However, only the 

 movements of whole leaves produced by upward and downward curvatures of 

 the petioles, or the lower parts of the laminse, have been investigated in detail; 

 and the following statements refer to these only. According to Batalin and 

 Pfeffer the movements in question are very evident in the following well-known 

 plants. Several Balsaminese {Impatiens noli-me-tangere), Chenopodiese, Atriplicese, 

 Solanese, Mimulus, Mirabilis Jalappa, species of Silene and Alsine, some Com- 

 positse, Malva rotundifolia, (Enoihera, Poriulacca, Linum grandiflorum, species 

 of Polygonum, Senecio vulgaris, Ipomaea purpurea (Blue Bindweed), Brassica 

 okracea (Cabbage); I may also add the leaves of Helianthus annuus (Sunflower), 

 and particularly the leaf-like cotyledons of many dicotyledonous seedlings. 



From the investigations of the observers mentioned, the movements of such 

 leaves agree with those of the group described in detail above, in all essential 

 points except one ; these leaves are not furnished with special motile organs, and are 

 found to be in periodic movement only so long, and are only so long para- 

 tonically sensitive for variations of light, as they are still growing, and in fact 

 the irritability begins to make its appearance when the young leaves come 

 forth from the bud ; the irritability increases, and the magnitude of the daily 

 movements likewise, in proportion as the growth is accelerated, in accordance 

 with the grand period of growth ; when the growth becofties slower again, the 

 irritability and motility also decrease more and more, until at length, on the 

 cessation of growth, the sleep-movements also disappear. Here, also, the motile 

 part, capable of curvature, need not remain the same; if the zone of strongest 

 growth advances along the petiole, or forwards from the base of the lamina, the 

 curvatures also occur at corresponding places. 



With regard then especially to the mechanism of this movement, practically 



