744 MECHANICAL LAWS OF GROWTH. 



been ascertained that, especially when long spaces of time such as entire days are 

 compared, all the other conditions of growth are outweighed by the effects of the 

 variations of temperature, so that in general the rapidity of growth increases with a 

 rising and decreases with a falling temperature. The result of a number of measure- 

 ments made by Rauwenhoff during several months in the most changeable weather 

 was that the mean growth was greater in twelve hours of the day than in twelve hours 

 of the night ; viz. — 



By day. By night. 



in Bryonia 59-0 p.c. 4i-op.c. 



Wistaria 57*8 42*2 



Vitis 55-1 44-9 



Cucurbita 567 43-3 



do. 57-2 42-8 



Dasylirion 55-3 447 



A similar tabular statement shows that the favourable influence of a higher 

 temperature by day outweighs the retarding influence of daylight. Rauwenhoff' s 

 measurements show accordingly that the mean growth during six hours of the fore- 

 noon is less than that during six hours of the afternoon ; since, while the average 

 amount of light is the same, the temperature is higher in the afternoon than in the 

 forenoon. If the afternoon growth is placed at 100, then the morning growth is — 



If however we calculate from Rauwenhoff 's measurements the daily and nightly 

 and the morning and afternoon values for shorter periods in which the changes of 

 the weather do not neutralise one another, it will be found that the growth by night 

 sometimes exceeds that by day, and that the afternoon is not always more favourable 

 than the morning. 



It is clear from what has been said that it is impossible to determine from 

 observations in the open air, where the variations of temperature, light, and moisture 

 are very great and are combined in a great variety of ways, in what manner each 

 separate condition of growth affects the plant, and whether the alternation of day and 

 night causes a similar alternation of growth, or whether there exist in the plant itself 

 causes of daily periodicity independently of external changes. In order to decide 

 this question, it is necessary first of all to make the observations independent of the 

 accidents of weather, which is only possible by carrying them on in well-closed 

 rooms where the temperature is kept constant or made to vary, and where the 

 amount of light can be increased or decreased, and the moisture regulated in the air 

 and in the soil of the flower-pot. Under these circumstances it is possible to study 

 the action of an increasing or decreasing amount of light upon a plant exposed to 

 constant conditions of humidity and temperature, and therefore exhibiting a con- 

 stant degree of turgidity ; it is sufficient to measure and compare the increments of 

 growth during short periods of time. 



