412 PHYSIOLOGY OF GROWTH. 



temperature as possible. From time to time, say every twenty- 

 four hours, we determine the aggregate growth of all the roots in 

 each cylinder, and we shall arrive at the result that the growth is 

 less in the light than in the absence of light. 



If plants are exposed to alternation of day and night, the tem- 

 perature and moisture being kept as constant as possible, it will 

 be found that generally the growth of their organs increases from 

 evening to morning, but diminishes from morning to evening. 

 This daily period of growth is the result of the variation in the 

 conditions of illumination during the twenty-four hours. In the 

 daytime the light retards the growth ; the darkness during the 

 night accelerates the rate of growth. Sachs l has, by means of 

 the auxanometer, determined the fact that there is a daily period 

 of growth in the internodes of various plants, and Prantl 2 has 

 similarly demonstrated the existence of a daily periodicity in the 

 growth of leaves. The experiments necessary are very tedious. 

 It is simplest to demonstrate the daily period of growth in leaves. 

 We employ for the purpose the methods described at the end of 154. 

 The plants (Cucurbita or Nicotiana) are placed under bell-glasses 

 in a room with a north aspect, and exposed to the alternation of 

 day and night, the temperature being kept very high and as 

 uniform as possible. From time to time (e.g. every three or four 

 hours) we measure with a millimetre scale the distance between 

 the apex of the leaf and the mark at its base, taking care not to 

 drag the leaf too much in laying it out flat. It then appears that 

 the growth is greater during the night than in the day. When 

 darkness comes on in the evening, the growth of the leaves is 

 not considerably accelerated at once, but quite gradually, so that 

 the maximum of the daily growth falls in the morning hours. 

 Similarly the daylight does nob at once reduce the rate of growth 

 of the leaves to the minimum ; the minimum rate of growth does 

 not appear until evening. 



Similar results are obtained if we determine the growth in 

 width of the leaves as well as their growth in length. The neces- 

 sary marks are made on the edges of the leaves, in the neighbour- 

 hood of the greatest diameter, and the distance between them is 

 determined at intervals of three or four hours. 



In researches on the daily period of growth of internodes, it is 

 very convenient to employ shoots of Dahlia variabilis grown in 

 the light. The tubers are placed in the soil, which is contained 

 in very large flower-pots, a good time before the beginning of the 



