PLANT HORMONES 225 



There are ample facts to prove the existence of formative influences 

 exerted by hormone-like substances, but we are far from understanding 

 the mechanism through which the chemicals act. It seems logical to assume 

 that the nucleus is the stable element, and that the "normal" form of the 

 plant results from this stable element actmg in combination ^\-ith the "com- 

 plex influences" from the cytoplasm. If one considers the cytoplasmic 

 influence the less stable, then it may be assumed that the modifications in 

 form may be brought about by the action of a chemical environment upon 

 the cytoplasm. Each chemical constitutes a different environment and 

 therefore permits different potentialities of the protoplasm to develop. "■*• "^ 



Other Subjects of Interest Involving Plant Hormones 



Only brief discussions are included. If details are needed, the citations 

 wall be helpful. 



Absorption and movement of growth substances. The controversial issues 

 on direction of movement of hormones were somewhat moderated when it 

 was demonstrated that roots can absorb gro^^h substances from the soil.i^ 

 The fact that the active substances were absorbed and translocated 

 throughout the plant was indicated by the response of the aerial parts. 

 The rate at which the substances were absorbed and moved in the plant 

 varied with the rate of transpiration, increasing with increasing transpha- 

 tion. Applied at the tip of the stem by means of sprays or lanolin prepara- 

 tions, the substances moved downward, causing a systemic response. 

 When applied to the middle of the stem the substances caused a response 

 above and below the treated region. In short, it was sho^\-n that growth 

 substances can move in all directions through the plant. 



Light and dark effects. Young gromng tomato plants maintain their 

 power to respond to growth substances while in light and for a period of 

 hours or days (depending on size and age) when transferred to a dark 

 room. They first lose their capacity to right themselves while in darkness, 

 when placed in a horizontal position. When this condition first occurs, 

 the plants may use synthetic substances to bring about tropic responses. 

 Later, however, young plants lose their power to respond to either natural 

 hormones or synthetic substances. Plants attached to storage organs 

 (potatoes, Jemsalem artichoke, etc.) or having storage tissue m the pith 

 (tobacco) mamtained their power to respond to growi:h substances for 

 more than 40 days after being placed m a dark room.^ 



Tropic curvatures. Natural tropic curvatures, such as bending of the 

 stem toward light or away from the earth, are assumed to be due to unequal 

 distribution of the natural hormones.^-- ^^ Stems treated unilaterally \\ath 

 preparations of groT\i;h substances underwent pronounced negative bend- 

 ing when in darkness or when light came from all directions. If, however, 

 the plants were allowed to make natural tropic responses before the sjm- 

 thetic growth substances were applied, the final responses were greatly 

 modified. When the substance was applied opposite the side that natural 



