GROWTH BF DAY AND NIGHT. 555 



must be more energetic than during the night. Day Hght, however, works in the 

 opposite way, and the question will therefore arise whether the intensity of the 

 effective rays suffices to nullify the effect of the temperature. The result also appears 

 to adapt itself according to the specific nature of the plant, since it is conceivable 

 that some plants are more sensitive to light than others. In the daytime, also, the 

 difference of tension of the aqueous vapour in the atmosphere is usually greater than 

 during the night, and transpiration is thus increased, and it may easily happen that 

 turgescence during the day is less than at night, and growth likewise may be 

 retarded thereby. The case might occur, therefore, that the growth during the day, in 

 spite of the higher temperature, was smaller than at night, and this will certainly be the 

 case if the temperature during the day is the same as that at night, or lower than that. 

 If, on the contrary, the excess of temperature of the day as contrasted with the night 

 is very considerable, it is probable that the effects of light and transpiration will be 

 overcome, and that the diurnal growth will remain more energetic than the nocturnal, 

 although the latter is promoted by the darkness and, as a rule, higher turgescence. 



' We may further notice yet a few more extreme cases which are here possible. 

 It might happen that the temperature at night was higher than that of the following 

 day, and that at the same time rain falling during the night increased the turgescence 

 to a maximum, while on the following day (the brightness being considerable) for 

 example a cold wind prevailed ; in this case the nocturnal growth would be the more 

 intense. In the Spring-time or in Autumn it may happen that the air sinks during the 

 night below the specific zero-point of the plant, so that the moisture and darkness are 

 unable to promote the growth ; cessation then occurs, and growlh takes place only 

 during the day when the temperature is raised sufficiently above the specific zero-point. 



' If we further suppose the external causes of growth so distributed that they 

 would not of themselves effect a too considerable difference of growth by day 

 and by night, then the difference may be exactly compensated or even reversed by 

 the different capacity for growth of the plant at different times, e. g. by the influence 

 of the phase of the grand period. For instance, if an observed internode at night, 

 when the conditions are otherwise unfavourable, has attained its maximum growth 

 (the apex of the grand curve), the growth on the following day may still be smaller, 

 though otherwise the conditions are more favourable. 



' These and numerous other combinations are possible, then, when we compare 

 only the average values of day and night. The number of possible cases is still 

 larger when the attempt is made to form an idea of the events from hourly obser- 

 vations. If we suppose the grand curve of growth of an internode to be draw'n, the 

 hourly alterations of temperature and the hourly alterations in the intensity of the 

 light, and of the psychrometric difference, will alter the course of the curve sometimes 

 in one way, sometimes in another: the curve, which during constant external 

 conditions ascends and descends in the form of a simple arc, will become transformed 

 into a much and variously zigzagged line, in the steps of which the daily and nightly 

 up-and-down oscillations of the growth are more or less clearly recognised ; the size, 

 form and position of these steps results for the time being from the co-operation of 

 temperature, moisture and light. 



'These indications will suffice to show of how little use it is for observers 

 to simply establish in what relation the nightly and daily growth stand to one 



