GROWTH AND MOVEMENT 



457 



Advantage. — The benefits of photeolic movements have been van- 

 ously imagined. They have been supposed to prevent injury to the 

 leaves by frost, since the folded position diminishes radiation; or to 

 prevent the formation of dew, so that transpiration may begin promptly 

 in the morning. The difficulty with the first of these ideas is that frost 

 does not occur in the regions where Leguminosae, which exhibit them 

 more strikingly than any other family, most abound; furthermore, a 

 temperature approaching o° C. would render response impossible. The 

 second explanation involves the assumption that transpiration is a valu- 



Figs. 688,689. — Autographic rec- 

 ords of leaf movements: dates and 

 hours of the day are given below; 12 

 noon, 24 midnight; the horizontal 

 median line represents the line the 

 recording point would have described 

 had the leaf remained quiet, moving 

 neither toward the diurnal {day) nor 

 the nocturnal {night) positions; the 

 black strips mark the periods of dark- 

 ening, which have no relation to the 

 natural alternation of day and night; 

 688, photeolic movements of leaf of 

 Alhizzia lophantha ; after a period of 

 constant illumination the plant was subjected to 6-hr. periods of alternating darkness 

 and light, then to continuous light, and finally to 3-hr. periods of alternate darkness 

 and light; note the persistence in light (Oct. 25-27) of the movements, which gradually 

 disappear; 689, photeolic and autonomous movements of leaf of Phaseolus vitellimcs^ 

 the latter restricted to the reversed periods of illumination (6 p.m. to 6 a.m.) ; note the 

 lag of the response in the former. — After Pfeffer. 



able function which the plant promotes, instead of a danger that 

 menaces its very life. It is dififiicult to conceive the significance of these 

 movements in terms of welfare, and it is quite possible that they have 

 none. > 



Other stimuli. — Changes in temperature, which often coincide and cooperate 

 with changes of light in producing photeolic movements, may act alone, and, when 

 sufficient, call forth like responses. Severe injury, even when wrought without 

 mechanical disturbance, as by burning with a lens, will also stimulate the motor 

 organs to curvature. So will a variety of other stimuli. 



