GROWTH AND MOVEMENT 
457 
Advantage. The benefits of photeolic movements have been vari 
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 ; 1 2 
noon, 24 midnight; the horizontal 
median line represents the line the 
recording point would have described 
had the leaf remained quiet, moving 
neither toward tb t diurnal (day) nor 
the nocturnal (> t'ght) 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 
Albizzia 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 vitellinus, 
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 promote?, instead of a danger that 
menaces its very life. It is difficult 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. 
