INTRODUCTION AND HISTORICAL SKETCH 11 



conduction of a stimulus over the incision is not proved beyond 

 objection. There remained the problem of demonstrating that 

 conduction of the stimulus can take place even when the living 

 connection between the tip and base is completely destroyed. 



Accordingly, two wedgelike incisions were made in the coleop- 

 tile about 1 cm. (or less) from the tip. Then the tip was removed, 

 and the upper part of the foliage leaf was taken off to about 2 mm. 

 above the womid. A small drop of gelatin solution was placed 

 upon the cut surface of the coleoptile stump, and the tip was 

 replaced in its original position, being held there by a ring of 

 cocoa butter. When this replaced tip was unilaterally illumi- 

 nated above the level of the cut, a decided positive phototropic 

 curvature appeared in the darkened basal region (Fig. 1). This 

 demonstrated beyond question that the stimulus could pass 

 over an incision. Conduction of the phototropic stimulus was 

 found to pass downward also when the cut surfaces were separated 

 from each other by a thin section of Calamus. Similar experi- 

 ments on the conduction of the stimulus in negatively geotropic 

 curvature in the Avena coleoptile were carried out with the same 

 result. 



From these investigations it became clear that conduction of 

 the stimulus in phototropic curvature takes place by the down- 

 ward movement of a growth-promoting substance upon the back 

 (shaded) side of the coleoptile. 



Paal. — The correctness of Boysen Jensen's work was questioned 

 by van der Wolk (1911). Later the experiments were repeated 

 and confirmed by Boysen Jensen and many other investigators 

 using somewhat different methods. Paal (1914, 1918) worked 

 with coleoptiles separated from the seed and from the primary 

 foliage leaf. After these empty coleoptiles had been placed in 

 damp sand, the tips were cut off by smooth incisions and then 

 replaced tightly. In about 88 per cent of the experiments a 

 stimulus conduction from the unilaterally illuminated tip to the 

 darkened basal region was demonstrated. Paal made the addi- 

 tional discovery that if the excised tip was placed on only one 

 side of the coleoptile stump (Coix), greater growth occurred on the 

 side beneath the replaced tip, and the coleoptile exhibited a 

 marked curvature (Fig. 1). He replaced a decapitated tip of 

 Avena in its normal position, inserting a piece of mica across one 

 half of the stump so that tip and base were in contact only on one 



