INSTANCES OF PHOTONASTIC AND DIURNAL MOVEMENTS 107 



of other responses to light stimuli, it is mainly produced by the blue and 

 more refrangible rays 1 . 



[The assumption that the * paraheliotropic * position is the result of 

 a heliotropic response is hardly justified, any more than is the assumption 

 that the pulvini of the main and secondary petioles possess the same 

 irritability and mode of response as those of the leaflets. The main 

 pulvinus of Mimosa pudica, and to a less degree those of the secondary 

 petioles, are, for in- 

 stance, heliotropic and 

 curve or twist under 

 unilateral illumination 

 even when compara- 

 tively intense, so as to 

 place the general sur- 

 face of the leaf more 

 or less at right angles 

 to the incident rays. 

 The folding-up of the 

 leaflets in strong sun- 

 light is, however, per- 

 formed in whatever 

 position the leaf may 

 be, and takes place 

 also when the leaves 

 are illuminated from 

 beneath by a beam of 

 light thrown upon one 

 or more of the pulvini 

 of the leaflets, each 

 pulvinus reacting 

 separately. The re- 

 lationships are some- 

 what complicated by 

 the fact that the pul- 

 vini of the leaflets also 

 appear to possess a weak heliotropic irritability ; but sufficiently strong illu- 

 mination, whatever its direction, always causes the same response, the 

 leaflets folding together owing to the reversal of their previous photonastic 

 response. In regard to the leaves of other Leguminosae, both the nycti- 

 nastic and the paranastic (paraheliotropic) positions of the leaflets are 

 produced, not in response to the direction of the illumination, but to its 



FlG. 31. Bauhinia tomentosa. A plant climbing by hook-tendrils, and 

 whose leaves show photonastic movements, (a) A leaf expanded owing to 

 the pulvinus being covered with tinfoil ; (b) the same but the pulvinus being 

 shaded by another leaf; (c) young leaf which has not yet unfolded: (a) 

 folded leaves with pulvini exposed to sunlight and showing that the folding 

 is independent of the direction of the incident light. (From a photograph. 

 After Ewart.) 



1 Ewart, l.c., pp. 451, 480; Macfarlane, Bot. Centralbl., 1895, Bd. LXI, p. 136. 





