896 
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 pratense 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. 
Pfeffer 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 effected by variations of turgidity, that is by the absorption 
and the escape of water. Since they are not accompanied, as Pfeff'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 different 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 II, with references to the literature. 
