448 The Physiology of Plants BOOK in 



In the next year Kraus found that the tissue strains in 

 growing organs attain a maximum at sunrise and fall to 

 a minimum at sunset ; and in 1880 he observed that the 

 greater growth of fruits and of fungi takes place in the 

 night. Friedrich's observations showed that the maximum 

 growth in thickness of stems takes place at night. Mil- 

 lardet, in 1869, showed a periodicity of the tensions in 

 Mimosa connected with the alternation of day and night. 



The induction of this periodic rhythm of growth upon 

 the constitution of the organism is supported by some 

 observations of Sachs in 1872. He found that induced 

 periodicity is to a certain extent independent of the main- 

 tenance of the conditions which set it up, for it persists 

 for some time when the plant is kept in darkness. Bara- 

 netzky confirmed and strengthened Sachs' results in 1879, 

 when he observed that a plant of Helianthus tuberosus 

 continued to show the rhythm during fourteen days of 

 darkness. He also found that when shoots were grown 

 from tubers of this plant in darkness, so that they did 

 not receive the periodic stimulation, they exhibited no 

 periodicity of growth. Godlewski made similar observa- 

 tions on certain seedlings in 1889. 



The influence of light on growth extends far beyond its 

 share in inducing this periodicity. The phenomena attend- 

 ing on growth in its absence, known long ago as etiolation, 

 were the subject of much research during the period 

 under review, and our knowledge of the abnormalities 

 which can be noticed in plants so cultivated, was very 

 widely extended. Sachs, in 1863, called attention to the 

 changes in relative sizes and position of the stems and 

 leaves and showed that these changes are not so uniform 

 as had been supposed. The hypocotyls of seedlings with 

 hypogeal cotyledons do not elongate in darkness, nor do 

 the stems of the hop, nor the lower internodes of those of 

 Bryonia. Certain obscure changes in the constitution of 



