10 THE EYE IN EVOLUTION 



flowers have a time of opening and closing so regular that he constructed 

 a flower-clock from which the time of day could be read — the poppy 

 opened at 6 a.m., the speedwell at mid-morning, the white campion in 

 the evening (to be pollinated by night moths), and so on (Figs. 5 and 6). 

 At a much later date, the " sleep movements " of leaves were similarly 

 studied by Darwin (1880) : those of the runner bean, for example, raise 

 themselves during the morning, become horizontal by noon, fall in the 

 afternoon and fold up at night. The significance of these daily 

 rhythms, however, was largely neglected until they were intensively 

 investigated by the German botanist, E. Blinning (1931-56), who 

 showed that they were not simply an immediate response to the 

 passing stimuli of day and night, but were part of a rhythmic change 

 which has become characteristic and endogenous to the plant itself — a 

 24-hour rhythm in the intensity of endosmosis throughout its structure, 

 in the rate of growth, the rate of respiration, the activity of enzymes and 

 the entire metabolism, a rhythm to which the plant has become 

 habituated so that the periodicity persists for some time even if it is 

 placed in continuous darkness, and is only slowly readjusted if an 

 artificial rhythm is imposed upon it.^. Other factors may supervene, 

 the most important of which are temperature and nourishment, but 

 the most profound influence on basic activities is that of the sun, from 

 the energy of which all life is ultimately derived. 



The pattern of the flowering of many plants is a good example of 

 this general tendency^ — and an important one, for floral initiation is a 

 fundamental factor marking the change from vegetative life to reproduc- 

 tive activity. Although experimental work of considerable merit had 

 been done on the effects of artificially varying the periods of illumina- 

 tion on the growth and maturation of plants, particularly by Schiibeler 

 (1880) in England, Tournois (1912) in France, and Klebs (1918) in 

 Germany,^ it was left to two American botanists. Garner and Allard 

 (1920), to establish finally the important fact that in many species 

 flowering did not depend primarily on temperature or the intensity of 

 illumination but on the daily lengths of the periods of light and darkness ; 

 they therefore introduced the term photoperiodism. In many plants 

 the determining factor is the length of the day, and, as was first proved 

 by the Russian botanist, Cailahian (1936), the primary receptor organ 

 is the leaf ; even although the rest of the plant is covered, the exposure 

 of one leaf, or even part of a leaf, to the rhythm of light and darkness 

 determines the cycle, and if the leaves are removed and the plant 

 rendered naked to live on its stored food it immediately becomes 



1 See Grossenbacher (1939), Engel and Heimann (1949), Flligel (1949), Hagan 

 (1949), Heimann (1950-52), Enderle (1951), Vegis (1955), Biinning (1956), Wareing 

 (1956), and others. 



" For review, see Smith, 1933. 



