Sg6 PHENOMENA OF SEXUAL REPRODUCTION. 



action of external influences has been already mentioned in Sect. 28, the move- 

 ment of the lateral leaflets of Hedysarum gyrans being cited as a striking example. 

 The rapid upward and downward movement of these leaflets take place, if the 

 temperature be sufficiently high, both in continuous illumination and in continued 

 darkness. The spontaneous movements of the leaflets of Oxalis acetosella and of 

 Trifolium praiense are also very evident ; the leaflets move in continued darkness and 

 at a constant temperature through an angle of from 30° to 90° in from i to 4 hours. 



It was pointed out in the foregoing paragraph that these spontaneous move- 

 ments are independent of the persistent effect of the stimulating influence of light. 

 Pfeff"er clearly made this out in the case of the large terminal leaflet of Hedysarum 

 {Desmodiuni) gyrans, which makes upward and downward movements of small 

 amplitude in short periods of time.. Whilst the daily periodicity (Sect. 30) is 

 manifested only for a short time in prolonged darkness, the spontaneous movements 

 continue. In Trifolium and Oxalis the persistence of the daily periodicity is very 

 slight, whereas the spontaneous movements are very evident in prolonged darkness. 



It can scarcely be doubted, after what has been already said with reference 

 to the various movements of growing and of mature organs, that the spontaneous 

 periodic movements are eff"ected by variations of turgidity, that is by the absorption 

 and the escape of water. Since they are not accompanied, as Pfefl"er has observed, 

 by any alteration of the rigidity of the organ, it is probable that one half of the 

 tissue gains in expansive force what the other loses and at the same moment ; this 

 almost amounts to saying that first one and then the other half of the tissue absorbs 

 water from the other half. 



Batalin (Flora, 1873) asserts that each periodic movement in Mimosa and Phaseolus 

 is accompanied by a slight increase in length. Simple calculation suffices, however, to 

 show that the periodic movements of these organs cannot be due to periodic growth of 

 the upper and under surfaces. 



The question why it is that the *two sides of the organ are alternately more or less 

 strongly turgid cannot be answered at present any more than the question why it is that 

 first one side and then the other grows more rapidly in growing leaves, stems, tendrils, 

 &c. which exhibit nutation. 



CHAPTER VI. 



THE PHENOMENA OF SEXUAL REPRODUCTION. 



Sect. 32^. Sexuality consists essentially in the formation, in the course of 

 development of the plant, of reproductive cells of two diff'erent kinds, which have no 

 independent power of further development, but which, by their coalescence, give 

 rise to a product which possesses that power. 



^ The facts upon which the considerations contained in this section are based are fully detailed 

 in Book 11, with references to the literature. 



