PERIODICITY OF DAY AND NIGIIT. 199 



of buds and flowers which lakes place usually in the ncxl spring. This vegetable 

 substance produced in the green organs is constructed from carbon dioxide and 

 water, with the separation of a ver}- considerable quantity of ox}-gcn ; it is therefore 

 a substance poor in oxygen. Plants, as is well known, are combustible, i. e. their sub- 

 stance, poor in oxygen, is again converted, as it burns in the air, into the compounds 

 rich in oxygen (carbon dioxide and water) from which it was originally produced 

 in the cells containing chlorophyll. Just as much heat as is set free in the burning 

 of a tree, must have been fixed during the production of its organic substance. 

 This heat of combustion of a plant, however, represents a definite amount of energy 

 which can be made useful — in a steam-engine, for instance : an amount of mechanical 

 work exactly as great, only in another form, was performed in the cells of the plant 

 containing chlorophyll during the production of the organic combustible substance 

 in them, from water and carbon dioxide with the separation of oxygen. Or, in other 

 words, the energy which is produced by the combustion of vegetable substance existed 

 originally in the form of luminous vibrations of the ether, the energy of which has 

 been employed in the cells containing chlorophyll for the separation of the oxygen. 



This work of the organs containing chlorophyll also changes with the alternation 

 of day and night ; but the resulting daily periodicity in vegetable life is made evident 

 in many other phenomena. In the first place, in a periodical growth of the young 

 organs, which may be accelerated by the darkness of night, unless the temperature 

 sinks too low, and brings about, on the contrary, a retarding of the growth: thus, 

 the rapidity of growth during the night may be greater or less than during the day\ 

 according to circumstances. To the daily periodic changes most easily recognisable 

 belong the so-called sleep-movements of leaves ; these are particularly conspicuous 

 in the compound leaves of the Leguminosese, e. g. Rohinia and the Oxalide» ; they 

 take place in such a manner that the parts of a leaf fold themselves together in the 

 evening, either upwards or downwards, and open out again on the following morning. 

 But even simple foliage leaves move. In Jul}- and August one need only look around 

 a garden after sundown to perceive the altered position of the leaves of almost 

 all the plants ; the large foliage leaves of the Sun-flower [Helianthus aiinmis), for 

 instance, all bend downwards, so that the upper ones in part cover the lower, 

 while the foliage-leaves of the large Balsam {Ijupatiens glandidiferci) all stand upright 

 at night. These are only examples, however, since these movements are quite 

 general. Still better known are the so-called sleep-movements of flowers, many 

 of which close before sundown to open again next morning — phenomena which 

 we shall consider in detail later on, and which depend upon the diurnal changes of 

 intensity of the light and temperature. I shall also have to speak more in detail 

 later on of the heliotropic curvatures of the growing parts of plants, and of the 

 influence of light on the hitherto unexplained swimming movements of the so-called 

 swarm-spores of the Algse. The manner in which light operates as a stimulus on 

 plants is exceedingly various, and by its periodic daily alternations it leads conse- 

 (juently to daily periodical changes in plants. 



■ I criticised, and corrected by my own observations, the older views (in great part quite 

 wrong) on the daily periodicity of growth in length, in my treatise, ' Über den Einßiiss der Lufttem- 

 peratur des Tageslichtes auf die stündlichen und täglichen Aenderungcn des Ldngenii.'achsthums'' in 

 Sachs' ' iVrbeiten des bot. Inst, in Wzbg.,' B, I, p. 99. 



