196 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS 



stimulation ? Under geotropic action the stem will tend to 

 curve upwards ; should it be positively phototropic, the curva- 

 ture under vertical light will also be upwards. Geotropism 

 and phototropism will thus conspire, the joint effect being 

 G + L ; but should the organ be negatively phototropic 

 the result would be G — L. If further account be taken 

 of the reactions of the organ to feeble and strong stimu- 

 lations of gravity and light, the following combinations 

 are possible : 



G+L; G-L; G+/; G-/. 



^ + L ; ^ - L ; g +l\ g ~l. 



Eight different effects can thus be produced by the 

 combination of only two factors ; there are, however, other 

 factors active, such as the rise and fall of temperature. 

 Additional complications are introduced by the unequal 

 sensitiveness of the two sides of the organ ; in some it is 

 the upper side, in others it is the lower side that is the more 

 excitable and therefore reacts more effectively. There are 

 thus at least ten factors in operation, and the different 

 combinations possible would exceed a thousand. 



It is therefore not surprising that the movements of 

 plants appear so extraordinarily complex. Efforts to dis- 

 cover a real explanation have long been baffled by the 

 fact that it has hitherto been impossible to isolate and 

 study the effect of each of the factors for the final analysis 

 of the complex result. 



In the consideration of the diurnal movements of plants 

 in general, the following subjects will be treated in detail : 



1. Daily movements in relation to light and darkness. 



2. Diurnal movements due to variation of temperature 



affecting growth. 



3. The diurnal movements of the ' Praying ' Palm. 



4. The effect of variation of temperature on fully grown 



organs subjected to the stimulus of gravity. 



5. After-effect of light. 



6. The complex diurnal movement of the leaf of Mimosa. 



