638 LECTURE XXXVI. 



is submitted to artificial illumination, in the manner just described for Tn/olium, 

 it is possible on the evening of the following day still to perceive clearly the 

 sinking corresponding to the after-effect of the daily period, the autonomous 

 movements going on all the time. The independence of this from the daily 

 periodic movements here comes sharply into view, and the same is the case when 

 the plant is kept in the dark, where it behaves much as in continuous light. This is 

 true also for Trifolium prafense — i. e. even on the first day of its being kept in the 

 dark the daily period is no longer perceptible, on account of the great amplitude of 

 the autonomous movements.' 



In agreement with the fact which I had previously established, that movements 

 in the vegetable kingdom which are induced by light depend upon the strongly 

 refrangible or so-called chemical rays, it is also found that the paratonic stimulation 

 of periodically motile leaves is caused by the light of the blue half of the spectrum. 

 I demonstrated so long ago as 1857 that in the case of the leaves of the Bean and Wood 

 Sorrel, when in their diurnal position, covering them with a dark blue glass bell-jar which 

 excludes all the yellow, green and orange light, produces no change whatever in the 

 position of the leaves, although to our sense of sight a strong darkening is connected 

 with it ; to the plant, however, it reacts not like darkness but like complete light. If 

 the plant, or a single leaf of it, is covered with a bell-jar of ruby coloured glass 

 which transmits only red and a trace of green light, then the effect is exactly as 

 if an opaque receptacle had been employed : the leaf in a short time assumes its 

 nocturnal position, and thus reacts to the red light as towards darkness. It 

 would be a mistake to suppose from what has been said above, however, that only 

 the plant covered with the blue cobalt glass would awake again next morning ; for 

 the one under the ruby-coloured glass does so also, and we know indeed that 

 it would do so even if it remained in profound darkness. 



Attacking at last the question as to what the first effect of the stimulus 

 properly consists in, which an alteration of the intensity of the light produces, we 

 must adhere to the conclusion already arrived at and drawn directly from 

 the facts (p. 634) that it concerns changes in the turgescence in the two halves 

 of the tissue of a motile organ, and that these under the prevailing circumstances 

 can only depend upon the addition and abstraction of water. Darkening thus 

 causes an increased flow of water into the whole motile organ, but more rapidly 

 into the one half than into the other ; increased brightness of the light, on the 

 other hand, must cause an abstraction of water from the entire organ, because 

 it becomes more flaccid, and this again in the one half more quickly than in 

 the other. This water, however, is inside the cells, surrounded by protoplasm 

 and cell-walls, and in part is also present in the protoplasm or in the cell-walls 

 themselves. Two possibilities thus present themselves. Either the light acts on 

 the imbibition-forces of the cell-wall, increasing them by darkening and diminish- 

 ing them by brightening ; or the variation of light influences directly the properties 

 of the protoplasm itself, and induces in this changes by which the turgescence 

 may be increased or diminished. For the first of these assumptions no certain 

 analogy is known, and on closer reflection it only leads to new difficulties. 

 On the contrary I shall show in the next lecture that we can analyse certain 

 phenomena of irritability of the organs of Mimoseae and others, so far as to say 



