204 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS 



plant-organs, I shall consider the three ideal types : (i) where 

 variation of light is the most important factor ; (2) where the 

 movement is principally due to differential growth under 

 variation of temperature ; and (3) where a fully grown tree 

 at an inclination to the vertical, and therefore subjected to 

 the stimulus of gravity, exhibits up or down movement 

 under variation of temperature. 



The determination of the isolated effect of any one 

 individual factor, difficult as it is, can be arrived at by a 

 process of judicious elimination. Thus the predominant 

 effect of light, of variation of temperature, or of geotropic 

 action can be inferred more or less from the following 

 observations. 



Predominant effect of light and darkness. — The obvious 

 method of observing the effect of keeping the plant in con- 

 tinuous darkness presents the difficulty that the tonic 

 condition of the plant is greatly depressed, resulting in the 

 abolition of its normal irritability under prolonged darkness. 

 The effect of withdrawal of light can therefore be satis- 

 factorily observed only for about two days in succession. 



The characteristic effect of light is very marked at two 

 definite times of the day, when light appears and when it 

 disappears. The average dawn is approximately at 6 a.m., 

 and the average sunset at 6 p.m. ; unlike the diurnal varia- 

 tion of temperature, which is gradual, the change from 

 light to darkness or from darkness to light is relatively 

 abrupt. Since the change of temperature and of light are 

 both connected with the appearance and the disappearance 

 of the sun, some difficulty arises in discriminating the effect 

 of one from that of the other. 



Light and temperature effects, — Light appears in the 

 morning, say, at 6 a.m. ; it becomes most intense at noon ; 

 after 4 p.m. the hght wanes and darkness sets in quickly 

 after 5 p.m. and remains persistent till next morning. The 

 course of variation of temperature is somewhat different. 

 The minimum temperature is attained at about 5 A.M. in 

 summer, and at about 7 a.m. in winter. The maximum 



