'626 LECTURE XXXVI. 



upper and lower sides of the leaves always stand in definite relation to this, it 

 results that the upward and downward curvatures of the motile parts take place 

 in a plane which may at the same time be regarded as a longitudinal plane of the 

 shoot-axis— i. e. the motile foliar organs approach the part of the shoot-axis 

 situated either above or below their origin, or, regarding the apex of the shoot as 

 the centre, we may say the movements take place in a centripetal or centrifugal 

 direction. This is particularly well shown in the case of flowers, which are in 

 fact metamorphosed shoots with very short axes : while in the case of ordinary 

 foliage-leaves we distinguish an up and down movement, we have in flowers to 

 do with an in and out movement. This rule, however, only applies to entire leaves ; 

 in compound leaves the individual leaflets move upwards and downwards with 

 reference to the common rachis, or they make at the same time twists directed 

 forwards or backwards. 



Although broad, thin, flat parts of leaves are not entirely exempt from sleep- 

 movements, it is nevertheless found that in the more exquisite cases, where the alter- 

 nation of diurnal and nocturnal positions is very marked, and the light-stimulus thus 

 acts with great energy, the organs or parts in question assume a more or less 

 cylindrical shape, as is particularly conspicuous in the motile organs of the Legu- 

 mimosse and Oxalideae, and true compound leaves generally. As a rule, a motile organ 

 of this kind is a more or less elongated cylinder consisting of succulent parenchyma, 

 in the axis of which runs a non-lignified and very flexible strand of vascular bundles. 

 Since the movements of these organs consist in up and down curvatures, it is obvious 

 that sometimes the upper, sometimes the lower side of the succulent envelope of tissue 

 must be lengthened, though the axial strand need not undergo either elongation or short- 

 ening, because it lies in what may be called the neutral axis of the motile organ. It 

 is sufficient if this strand is flexible and not rigid ; its length need not alter during the 

 movement. Where the up and down curvatures of foliage-leaves are eff"ected by 

 means of ordinary petioles, or parts of the laminae, of course vascular bundles traverse 

 these parts as usual ; but they also abound in succulent parenchyma, and the movement 

 continues only so long as no lignification has as yet occurred in the vascular bundles. 

 These points are especially to be borne in mind with respect to the mechanics 

 of such movements, to be described more in detail subsequently. However, before 

 we go into the mechanics of the movements, it is necessary for us to become some- 

 what better acquainted with the movements of the leaves themselves. 



Our present theme is the so-called sleep-movements, caused by the daily varia- 

 tion of the light at sunrise and sunset. Now we shall at once see that even these 

 movements for and by themselves are combined of two kinds of actions, namely of a 

 direct stimulation, and of after-effects induced by the stimulus itself. It was, however, 

 by no means easy to establish this apparently simple fact, since other movements are 

 combined in the most various ways with the proper daily periodicity, though they 

 have absolutely nothing to do with it. This is very evident when such a plant as a 

 Mimosa, Bean, Oxalis, &c. is suddenly excluded from the light : there then occurs at 

 once, in fact, a sleep-movement ; in from half an hour to an hour the leaves assume 

 the nocturnal position. On again looking at it the next morning, however, the leaves, 

 although they have not received any light at all, are found to be in their diurnal 

 position, with their laminae extended : next evening they fold up in the nocturnal 



