CHAP, x The Problems of Growth 447 



and hyponasty to express these relations. Darwin pointed 

 out in 1880 that these phenomena really constitute a very 

 slow rhythm of nutation, and Vines in 1886 described 

 many instances of this rhythm ; Darwin pointed out that 

 in the case of the hypocotyledonary axis it is of great 

 use in the processes of germination, helping the seedling 

 to emerge from the seed coats and to make their way 

 through the soil. Wortmann pointed out similar purposes 

 in the case of epicotyls in 1882. Wiesner described a 

 modified form of epinastic growth exhibited by the epi- 

 cotyl under the term undulating nutation in 1883. Sachs, 

 Wortmann, and Vochting, showed that these modified 

 nutations of the axis of the seedling are spontaneous and 

 not dependent upon gravitation. In 1886 Vines reaffirmed 

 the rhythmic character of epi- and hyponastic growth. 



The normal conditions of growth and their relation to 

 external stimulation were discussed by Sachs in 1863, and 

 at some length by Pfeffer in 1893. The influence of light 

 upon them has been the most studied. We have seen that 

 a daily periodicity of growth in length was noticed by 

 several observers. A similar rhythm has been detected 

 in the case of growth in thickness. It was first observed 

 by Reinke in 1876, and later by Friedrich in 1897. Prantl 

 in 1873, and Stebler in 1878, showed that the leaves of 

 many plants show a daily periodicity of growth ; Kraus, in 

 1880, observed the same thing in the case of certain fruits 

 and of various fungi ; while McMillan, in 1891, and Golden, 

 in 1894, detected a similar periodicity in the growth of 

 the tuber of the potato. 



Pfeffer suggested that these periodicities may have been 

 induced by the alternation of day and night. A certain 

 correspondence between the two phenomena appeared 

 from the investigations of several botanists. In 1866 

 Duchartre found that the growth in length of various 

 shoots is greater during the night than during the day. 



