DAILF PERIODICITY OF GROWTH IN LENGTH. 745 



A long series of observations of this kind on internodes has given me the fol- 

 lowing results^ : — 



(i) The more exactly a constant temperature is maintained in a constantly- 

 dark space, the amount of moisture being also constant, the more uniform is 

 the course of growth at different periods of the day. There does not appear to be 

 any daily periodicity of growth independent of external influences. The irregular 

 variations of growth mentioned above were however observed. 



(2) If great variations of temperature are allowed to act on a plant growing 

 in darkness and with a constant amount of moisture, to such an extent that the tem- 

 perature of the air round the plant alters some degrees C. from hour to hour, 

 the rate of growth of the internodes rises and falls with the rising and falling 

 temperature. If the hourly increments are taken as ordinates, and the intervals of 

 time as abscissae, the curve of growth follows all the elevations and depressions of 

 the curve of temperature, without however any actual proportion being observable 

 between the growth and temperature ; the curves do not run parallel but are only 

 of the same description. 



(3) If care is taken that during the period of observation the temperature 

 undergoes only slight and gradual changes, while (the temperature being sufficiently 

 uniform) the amount of light changes in the ordinary manner, increasing from morn- 

 ing till midday and decreasing from midday till evening, to complete darkness at 

 night, it will be found that the increments of growth are always greater from even- 

 ing till sunrise, diminishing suddenly after sunrise, and then more slowly till evening. 

 The alternation of day and night causes therefore under these circumstances a 

 periodical rising and falling of the curve of growth of such a nature that a maxi- 

 mum occurs in the morning at sunrise and a minimum before sunset. A second 

 rising of the curve of growth usually takes place also in the afternoon ; but this, as 

 I have shown, is a consequence of the higher temperature in the afternoon which 

 overcomes the influence of light. The retarding influence of light is therefore 

 strong enough to overbalance the favourable influence of the slight elevation of 

 temperature in the forenoon, but not sufficient to overcome that of the stronger 

 elevation of temperature in the afternoon. 



The fact is of great interest that when a plant has been exposed to light during 

 the day, its curve of growth after sunset, or if placed in the dark in the evening, does 

 not immediately rise abruptly; i.e. that the most rapid growth which is independent of 

 light is not at once attained when it is suddenly placed in the dark ; but that — as is 

 shown by the curve rising slowly till morning — the growth which has been retarded 

 during the day only becomes gradually more rapid in the course of some hours, 

 until the light to which the plant is again exposed in the morning causes a fresh 

 retardation of growth, which again increases from hour to hour till the slowest rate 

 is attained in the evening, if the temperature remains constant. In other words, the 

 two internal conditions of the plant w^hich correspond to darkness on the one hand 

 and to daylight on the other hand pass over only gradually into one another. Light 



^ Sachs, Arbeit, des bot. Inst. Wiirzburg, 1872, p. 168 et seq. The plants observed were 

 chiefly Fritillaria imperialis, Humulus Lupulus, Dahlia variabilis, Polemonium reptatis, and Richardia 

 ccthiopica. 



