GROWTH MOVEMENTS 1087 



supplies an adequate explanation of this phenomenon. The view that 

 phototropic curvatures result from the direct action of an external agent 

 upon the growing region is held preferable to the interpretation of the 

 phenomenon in terms of a stimulus, or "releasing mechanism," as held 

 by Pfeffer. Blaauw's results indicate a close quantitative relationship 

 between the unilateral light causing phototropism and the response itself. 

 Priestley points out "that if the duration of exposure is sufficiently long, 

 any intensity of light may produce a phototropic response. The product 

 of minimal time of exposure necessary into intensity of light always gave 

 the same value of 20 meter-candle-seconds, below which light quantity 

 no response can he observed. Thus there is no absolute threshold value of 

 duration of exposure, or of intensity of light." 



Priestley emphasizes the point that etiolated shoots are far more 

 sensitive to light than normal ones and that this distinction must be fully 

 recognized in the interpretation of phototropism from his point of view. 

 Where phototropic bending occurs in normal light-grown shoots, light 

 influences the nutrition and extension of the cells at the growing point. 

 This mechanism is quite different from that occurring in etiolated shoots. 

 Here Priestley brings to bear his earlier observations (34, 35) in support 

 of Blaauw's views. In etiolated shoots "fatty and protein substances 

 will be released from complex forms of chemical combinations; the pro- 

 teins will disappear from the walls and the fatty substances migrate 

 mainly to the cuticle. The walls of the cells of the meristem, and of the 

 tissues which intervene between the meristem and the vascular supply, 

 are thus partly freed from protein and fat and now consist mainly of 

 carbohydrates. Along the walls the sap from the vascular supply can 

 now percolate more freely and water-soluble solutions in this sap find 

 readier access to the superficial cells of the meristem, which are farthest 

 away from the vascular supply, and which before were only growing 

 slowly. Increased superficial growth now ensues. Growth as a whole 

 may be as active as ever on the more brightly lit side of the etiolated shoot, 

 but it is differently distributed. More cells are added to the surface of 

 stem and leaf and less proportionately to the inner layers of the shoot 

 axis. The result is, therefore, in the aggregate, a retardation of growth 

 in length on the illuminated side and a positive phototropic curvature." 



In a later paper Priestley (37) reexamines the data found in the litera- 

 ture on phototropism of etiolated coleoptiles and attempts to use this in 

 support of his viewpoint rather than another for which they were intended. 

 It is pointed out that practically all coleoptiles employed in phototropic 

 experiments are at least 0.5 cm. long. At this stage the meristematic 

 tissue has disappeared, so that subsequent growth takes place through 

 vacuolation and extension in length of already existing cells and not from 

 cell formation. Cell extension is more rapid in light than in darkness, 

 but their final length is less. Priestley makes use of the fact that growth 



