500 TRANSFORMATION OF ENERGY ' 



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LECTURE XXXIX 

 NYCTITROPISM 



Many plant organs, especially foliage and floral leaves, take up, towards 

 evening, positions other than those which they occupy by day. Petals and 

 perianth leaves, for example, bend outwards by day so as to open the flower, 

 and inwards at night so as to close it. Corresponding movements take place in 

 entire inflorescences as, for example, in those of theCompositae ; the capitulum 

 may be said to open when the ray flowers, or all the flowers, of the head curve 

 outwardly, and to shut when they curve inwards towards the centre. Many 

 foliage-leaves, also, may be said to exhibit opening and closing movements, not 

 merely when they open and close in the bud but also when, arranged in pairs on an 

 axis, they exhibit movements towards and away from each other. In other cases, 

 speaking generally, we may employ the terms night position and day position for 

 the closed and open conditions respectively ; thus, for example, the umbels of 

 Daucus are directed vertically downwards in the evening and stand erect by 

 day. The night position may also be described as the sleep position. 



Day and night positions may arise by the combined action of geotropism 

 and heliotropism. Thus Vochting (1888) observed in the case of Malva verti- 

 cillata, that the leaves, when illuminated from below, turned their laminae down- 

 wards during the day, but during the night became erect geotropically. The 

 sleep movements in leaves and flowers referred to above cannot, however, be 

 explained by assuming such a combination of heliotropism and geotropism, 

 for, as a rule, they have nothing to do with tropisms at all, although they 

 are frequently occasioned by light or, in other cases, by heat. Light and heat 

 do not operate in these curvatures as they do in heliotropism and thermotropism 

 proper, where opposite sides of the plant are subjected to stimuli of unequal 

 intensity ; while in heliotropism and thermotropism we have to deal with a 

 variable regional distribution of heat and light, in the present case we have to do 

 with movements which are induced by these same factors operating at different 

 times. In other words, the plant reacts to variations in the degree of illumina- 

 tion and variations in temperature, and its reactions are not tropistic in character 

 but nastic ; we might, in fact, describe these opening and closing movements as 

 thermonastic and photonastic respectively, and might, at the same time, 

 characterize, with greater accuracy, the outward curvature as the result of 

 epinasty, and the inward curvature as the result of hyponasty ; that is to say, the 

 movements are photoepinastic or photohyponastic. These same movements 

 have also been described as nyctitropic and the phenomena as those of 

 nyctitropism. Although this phrase has now come into common use, we must 

 nevertheless point out that it is incorrect from two points of view; first, because 

 we are not dealing in these instances with a iropism at all, and, secondly, be- 

 cause it is not darkness (w'^) that is the releasing stimulus, but the alternation 

 of light and darkness. 



