MOVEMENT IN PLANTS. 255 



(d) Note carefully that (1) if the stimulus has been due to touch 

 with a pencil or needle, or the placing on the stigma of pollen from 

 a quite different pla,uf(e.g. Foxglove, Snapdragon, Plantain, or any 

 other plant from which pollen can be obtained and scraped on to 

 the Mimulus stigma), the lobes open and do not close again ; and 

 that (2) if pollen of Mimulus itself is placed on the stigma, though 

 the lobes open again as usual, after from two to three hours a 

 second closing occurs, and this lasts for about twenty minutes if a 

 small amount of pollen has been used if much pollen is applied, so 

 as to cover the inner faces of the lobes, the second closing is per- 

 manent. 



(e) Make experiments to show that (1) the question whether the 

 lobes shall remain closed or will reopen depends on the quantity of 

 pollen applied (whether of the same or of an alien species) to the 

 stigma ; (2) dry powdered starch or dry sand will cause prolonged 

 closure ; (3) withdrawal of water from the stigma tissue, by placing 

 the stigma in salt solution, causes prolonged closing. The results 

 will show that, while the instantaneous first closing movement is 

 due to contact irritability, the permanent closure results from the 

 prevention of the automatic reopening movement, and is evidently 

 due to (at least in part) absorption of water from the stigma tissue 

 by the pollen-grains and their tubes. 



The sole advantage of these remarkable movements of the stigma 

 lobes (which occur in species of Martynia and Torenia as well as in 

 Mimulus) appears to be that the germination of the pollen is 

 favoured by the formation of a "moist chamber "in which the 

 pollen can germinate more rapidly ; when pollen is placed carefully 

 on the stigma, shock being avoided so that no movement results and 

 the lobes remain open, the pollen grains germinate much more 

 slowly. The lobes remain closed only if the pollen grains and the 

 germinating pollen-tubes can by abstraction of water prevent the 

 return of the original osmotic pressure in the stigma tissue and the 

 consequent reversal of the primary closing movement, until a 

 sufficient number of pollen -tubes have penetrated the conducting 

 tissue and so disorganised it (by some chemical process) that reopen- 

 ing is made impossible. 



X. " NYCTITROPIC " MOVEMENTS. 



360. In many plants the foliage and floral leaves assume 

 in the evening positions other than those they occupy by 

 day. The movements concerned have been called nycti- 

 tropic, though as a rule they are not tropistic movements 

 but nastic movements, and the stimulus is the alternation 

 of light and darkness. Similar movements may also be 



