43 



and twisting of etiolated parts, since the activity of these cells 

 results in a displacement of the mechanical tissues. 



Perhaps the most important result reached by the author is the 

 demonstration of the absence of a paratonic action of light on 

 growth. The failure of a large proportion of the plants to mani- 

 fest an increased growth in darkness can only be interpreted as 

 demonstrating that there is no invariable relation existing between 

 light and increase in length and thickness or between the division 

 and increase in volume of the cell. On the other hand those 

 forms exhibiting a marked acceleration in growth when removed 

 from the light show adaptional elongations that can be explained 

 by the stimulative action of darkness rather than by the retard- 

 ing action of light. The views of Sachs on this subject and on 

 the morphological significance of climbing plants and in fact the 

 entire views of his school on the relation of light and darkness 

 to growth and development fall to the ground as a result of the 

 evidence here brought forward. Several other widely accepted 

 views become untenable in the light of these extended and accur- 

 ately performed experiments. The attempt by Kraus to explain 

 the atrophy or meager development of leaves on the basis of a 

 lack of nutrition is seen to be futile when wider observations 

 reveal the fact that often the development in darkness may equal 

 or exceed the normal growth. ' So also the belief, generally ac- 

 cepted since the time of Boehm and Godlewsky, that the elonga- 

 tions manifested in etiolated plants are adaptive reactions to lift 

 the photosynthetic parts into the light is overthrown by the 

 results that were obtained in many instances where these organs 

 either failed to show any response that could possibly be in- 

 terpreted as adaptive or on the other hand were clearly the 

 reverse of beneficial. The main conclusions of the author may 

 be summed up in these words : Darkness deprives the plant of 

 the determinative and morphogenic influence of light and conse- 

 quently the embryonic tissues are chiefly developed while the 

 secondary tissues that appear in the plant body, in the formation 

 of the flower, maturation of the fruit, etc., are poorly differen- 

 tiated. The growth, consequently, of the etiolated plant is due 

 to the stimulus of darkness and entirely controlled by its auto- 

 tropic and geotropic reflexes. Carlton C. Curtis. 



