Comparative Effects of Growth Substances on 



Stem Anatomy 



B. ESTHER STRUCKMEYER 



IX/Tany different growth substances have been tested in attempts to 

 determine the nature of their effect on plant growth (lo). It has 

 been found that growth substances may have characteristically different 

 effects on a plant, particularly on the anatomical structure of the stem. 

 Thus, in studies carried out here, 2,4-dichlorophenoxyacetic acid (2,4-D) 

 stimulated cell proliferation and lateral root formation (9); a-naph- 

 thaleneacetamide induced cambial activity resulting in the formation 

 of more xylem and in a thickening of the cell walls of the xylem and the 

 phloem fibers (4) ; and jS-naphthoxyacetic acid was particularly effective 

 in producing parthenocarpic tomato fruits (7). Alpha-naphthaleneacetic 

 acid has been reported to prevent the preharvest drop of apples in the 

 fall as well as to thin apples shortly after blossoming in the spring (3,8). 



In our studies of the anatomical structure of plants treated with 

 growth substances the following problems have been given special at- 

 tention: i) why do most dicotyledonous and only some monocotyledon- 

 ous plants respond to 2,4-D; 2) what is the possible role of a-naphthalene- 

 acetamide in delaying calcium deficiency symptoms; 3) and how does 

 a-naphthaleneacetic acid function in the thinning of apples at one stage 

 and in preventing them from dropping at a mature stage? 



Much work has been done on 2,4-D regarding its practical value as a 

 weed-killer. It has recently been reported that some species of dicotyle- 

 dons and monocotyledons responded to 2,4-D while others did not (2,6). 

 The structure of the stem of young kidney bean plants treated with 2,4-D 

 was reported by Swanson (9). In our investigations older plants were 

 used (six internodes or more) belonging to the monocotyledonous and 



