Expandmg Concepts 15 



the cotyledons, so that unless the upper part first became bent, the 

 lower could not bend, however much it might be stimulated. It was 

 necessary for our pmposes to ascertain whether this notion was true, 

 and it was proved false; for the lower halves of several cotyledons be- 

 came bowed to the light, although their upper halves were enclosed 

 in little glass tubes (not blackened) which prevented, as far as we 

 could judge, their bending. Nevertheless, as the part within the tube 

 might possibly bend very little, fine rigid rods or flat splinters of thin 

 glass were cemented with shellac to one side of the upper part of 15 

 cotyledons; and in six cases they were in addition tied on with 

 threads. They were thus forced to remain quite straight. The result 

 was that the lower halves of all became bowed to the light, but gen- 

 erally not in so great a degree as the corresponding part of the free 

 seedlings in the same pots, and this may be accounted for by some 

 slight degree of injury having been caused by a considerable surface 

 having been smeared with shellac." 



But before you read Darwin's account of his experiments, may 1 

 suggest that you visit his home, Down House, near the tiny village 

 of Downe, some 20 miles from London where he and his sons, Francis 

 and George, did the experiments about which he writes so engagingly. 

 There you can see the remains of the simple greenhouses in which 

 his plants were grown, his study, still much as it was when he used 

 it, and the Sand Walk where he daily took his constitutional and re- 

 viewed the experiments he and his sons had made, and planned new 

 ones. 



Darwin did not express an opinion on how the stimulus of light 

 was transmitted from the tip to the base. He was inclined to consider 

 changes in turgescence as responsible for the movement but says, "In 

 what manner light, gravitation, etc., act on the cells is not known." 

 During the next 30 years investigators confirmed and extended 

 the major observations and conclusions Darwin had made. The 

 sensitiveness to light was defined in terms of light units, the sensitive 

 area was more clearly limited, the effective wave lengths were studied, 

 equipment for subjecting coleoptiles to light of known intensity and 

 composition was devised, but the fundamental question — how was 

 the stimulus transmitted from the tip to the coleoptile base — re- 

 mained unanswered. Did light induce electric currents which, mov- 

 ing downward, caused unequal growth and bending? Did light 

 induce changes in electric potential, changes in turgor, in permeabil- 

 ity, in sap reactions which were propagated downward? Or did it 

 cause the formation of growth inhibitors, destroy growth acceler- 

 ators, influence polarity, modify the movement of food, or water, or 

 act in some ill-defined way? 



