504 



TRANSFORMATION OF ENERGY 



carried out only whilst the leaf is still growing ; the amplitude of the 

 movements decreases proportionally with the age of the leaf. Other leaves, 

 however, retain the power of nyctitropic movement even in the full-grown 

 condition, but, as is to be expected, they are distinguished by the presence of 

 articulations at their bases. As to the special capabilities of the articulation, we 

 have already learned something when treating of heliotropism and geotropism. 

 Most, but by no means all, leaves which are provided with articulations ex- 

 hibit nyctitropic movements, such, for example, as numberless Leguminosae, 

 many Oxalidaceae and Marantaceae, several Euphorbiaceae (Phyllanthus), 

 Zygophyllaceae (Porliera), and Hydropterideae {Marsilia), and many others 

 (Hansgirg, 1893, p. 131). In most cases, not one only but several articulations, 

 are concerned in the movement ; in bipinnate leaves, for example, we find one 

 pulvinus at the base of the main petiole, another at the base of each of the 

 secondary petioles, and yet another at the base of each individual leaflet, and 



when all these pulvini 

 operate at the same time 

 the movement of the leaf 

 which results must neces- 

 sarily be complicated. In 

 Mimosa pudica, for ex- 

 ample, in the day position 

 the chief petiole forms with 

 the stem an angle of about 

 60° upwards ; there are 

 two pairs of secondary 

 petioles present, the basal 

 pair of which stands almost 

 at right angles to the pri- 

 mary petiole, whilst the two 

 apical ones form an angle of 

 about 60° with each other 

 forwards ; finally, the leaf- 

 lets spread out horizontally, 

 and form angles of 90° with 

 the secondary petioles in 

 the same plane. The posi- 

 tion of the leaf at night is 

 entirely different,for the pri- 

 mary petiole bends down- 

 wards as much as 80° to 

 100°. The four secondary 

 petioles bend forwards so as to place themselves almost parallel with the long axis 

 of the chief petiole ; the leaflets bend upwards and approximate in pairs, their 

 upper sides touching. At the same time they suffer slight twisting, and now form 

 an acute angle forwards, with the secondary petiole, the basal ones overlapping 

 the apical ones like the tiles on a roof. The day and night positions are sketched 

 at Fig. 159, in Lecture XL. In most cases it is only the articulations directly 

 associated with the leaflets that exhibit these movements clearly ; the leaflets 

 are in Mimosa turned upwards at night, but in many other plants they are turned 

 downwards. The behaviour of the leaflets in Acacia, in Hypocrepis, and also in 

 Coronilla (Fig. 158, ///) is perfectly similar to that of the leaflets in Mimosa, save 

 that in the last case the leaflets are directed backwards instead of forwards. In 

 Trifolium the terminal leaflet is simply raised up while the lateral leaflets are not 

 only raised, but at the same time undergo a torsion of about 90°. A single down- 

 ward curvature in the articulation occurs in Rohinia, Amicia, Phaseolus, and in 



Fig. l.<;8. /, Desmodium gyrans in day position. //, The same in 

 night position. ///, Coroniila rosea in sleep position. After DARWIN, 

 (1881). 



