304 



METAMORPHOSIS 



The formative influence of continuous darkness has been longest known 

 and is the most striking of the formative effects. Apart altogether from the 

 alteration in colour which frequently takes place, plants which are grown in 

 darkness exhibit special peculiarities in shape which are summed up under the 

 term ' etiolation '. 



The pure etiolating effect of darkness is seen naturally only when light is 



i n 



Fig. 88. Dahlia variabilis. /, grown in light ; //, grown in darkness. From a photograph. Equally reduced. 



withdrawn, other factors remaining constant. One of the indirect results of 

 placing a green plant in darkness is to cause a cessation of carbon-dioxide assimi- 

 lation and a constant stoppage of nutritive supply. In studying the various 

 phenomena of etiolation, therefore, we must arrange that no such absence of 

 nutrients takes place, and hence we employ plants for dark cultures which are 

 abundantly provided with reserves (e. g. seeds, tubers, trees). If now we com- 

 pare an etiolated shoot of Dahlia variabilis (Fig. 88, //) with one grown in light 



