262 The Plant World. 



effect of compressing those cells nearer its outer surface in the 

 manner, but not to the extent, that the basal cells on the concave 

 side of the hair of Dionaea are by flexure of it. Indeed this may 

 be said to be true of all the cells, in reduced degree, approaching 

 the stigmatic surface, but we do not therefor remove the difficulty 

 of explaining the effectiveness of stimulus in one direction and 

 not in another. It appears more satisfactory at the present mo- 

 ment to regard the local stretching of the sensitive cells as the 

 critical condition which calls out a response. 



THE EFFECT OF POLLEN 



In the third English edition of the Bonn text-book (p. 296) 

 the statement is made that "the stigmas of Mimidus and Torenia 

 after closing on mechanical stimulus open again shortlv unless 

 active pollen has been deposited, \\ hen they remain in a closed 

 position." This is evidently based on the work of Burck, (/. c.) 

 of which a somew hat more s; ecilic account is given bv Jost *. 



Burck, working -with the above mentioned forms, came to 

 results of very great interest in this connection. In Torenia, the 

 stigma-lips close on lollination only after mechanical stimulus 

 and remain closed only if they have leceived pollen from the two 

 long stamens. If that from the shorter stamens, or from other 

 genera is used, the lips reopen. In Mimulus pollen from any of 

 its stamens ensures a closed condition. Burck found, however, 

 that in Torenia if the pollen from the shorter stamens be dried, it 

 becomes equally effective with that from the longer ones, or that 

 if from the latter be first moistened, it becomes non-effective. 

 The conclusion drawn by Burck is that the effect of the pollen is 

 to withdraw water from the stigma cells, thus reducing their tur- 

 gor and causing closure, f The movement of the stigma pro- 

 duced by the action of pollen is therefore not to be regarded as 

 an expression of irritability, any more than the closure of the lips 

 if effected by dry heat, as e. g., when a hot glass rod be held near 



♦Jost Lectures on Plant Physiology, p. 520. 



♦In the review, to which I alone have access, (Bot. Centralbl. 89: 645. 1902) the word 

 ' 'Nnrbenschleim ' ' is used. I do not find that a mucilage is present on the stigma in nplacus 

 or Mimulus, and I believe that this word is unfortunate. 



