( 334 ) 



Therefore it is again easilj' miderslood why Boysen Jensen 

 obtained no phototropic cnrvature of the base in air satnrated 

 with water vapour when he introduced a mica plate into the posterior 

 incision. It always takes some time to introduce a mica plate into 

 the \evy fine incision, wiien working in a weak red light with an 

 average of 8 — 10 seedlings, and taking care not to handle the objects 

 more than is necessary ; above all great care is necessarj' in placing 

 the little cylinders round the bases. The result of Jhis is, that the 

 seedlings huve already been exposed too long to the air of the room 

 to give a phototropical curvature of the base, because the influence 

 of the evaporation from the wound has alreadj^ become much too great. 

 This is my reason for repeating Boysen Jensen's experiments in such 

 a way, that special precautions were taken to avoid making the cut 

 needlessly large and that the seedlings operated on were only exposed, 

 to the influence of the air of the room for a minimal time. 



For this purpose each seedling is treated separately and finished 

 completely ; the cut on the posterior side is only made in the coleoptile 

 and in particular does not extend (o the leaves since these by an 

 evaporation curvature counteract any curvature which might occur 

 in the base of the coleoptile. Hence a semi-circular cut is made, into 

 which a crescent-shaped piece of tin-foil is introduced, so that the 

 wound is really completely shut olf. Next the base is covered with 

 a little cylinder of black paper and the coleoptile so treated is at 

 once covered by a bell-jar saturated with water vapour. Thus, from 

 a box of seedlings, each coleoptile is separately worked through ; 

 specimens, in which the operation does 'not succeed readilj-, are 

 definitely excluded. When all the seedlings have been thus treated, 

 unilateral illumination is admitted. 



The result of this experiment was according to expectation, but 

 nevertheless equally surprising and convincing -. for in these conditions 

 the base actually executed a clear phototropic curvature. With this 

 not only is the interpretation of Boysen Jensen disproved, but it is 

 moreover shown that his results must really be ascribed to some 

 influence of the evaporation from the wound. 



But also in a quite different way, I have succeeded in showing 

 the inaccuracy of Boysen Jensen's interpretation. As has already 

 been mentioned, when a cut is made on the posterior side, the base 

 shows a strong curvature towards the wound. Yet if the apex is 

 illuminated on its anterior side then the base remains erect, and 

 does 7iot curve towards the wound as indeed Boysen Jensen found. 

 The question which at once presents itself is ivhy, in this last case, 

 the base does not curve towards the wound, for if Boysen Jensen's 



