824 
MECHANICS OF GROWTH. 
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 various 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. 41*0 p.c. 
Wistaria 57-8 42-2 
Vitis 55*1 44-9 
Cucurbita 56*7 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 — 
in Bryonia 86 ' 
Wistaria 71 - . - '^ 
Vitis 67 
Cucurbita 79 ^ 
do. 81 ; , ■ 
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 can be 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. 
