IRRITABILITY. 



4OI 



The daily period of growth in length was first thoroughly 

 studied by Sachs, and we may take some of his results to 

 illustrate our present consideration of the subject. 



6pm 12, 



pm 12 



FIG. 45 (after Sachs). Curves of the daily period of growth in length of the stem 

 of Dahlia variabilis. \h is the curve constructed from hourly observations, 

 and $h is the curve constructed from observations made every three hours ; 

 * is the curve of temperature. The abcissae represent periods of two hours, 

 and the divisions of the ordinates represent units of increment in length. 

 With regard to the temperature, the base-line indicates i2R., and five of the 

 divisions of the ordinates correspond to i R. The shaded spaces represent 

 periods of darkness. 



A general inspection of the curves, particularly of the 

 curve constructed from observations made every three hours, 

 shews, as might be expected, that the rate of growth increases 

 during the night, and diminishes during the day. It will be 

 noticed that the effect of exposure to light or to darkness does 

 not manifest itself suddenly, but that in the one case the curve 

 gradually falls and that, in the other, it gradually rises. 

 V, 26 



