CHAP. VIII. ON CLEISTOGAMIC FLOWERS. 337 



pollen-grains generally emit their tubes whilst still 

 enclosed within the anthers ; but this is not so re- 

 markable a fact as was formerly thought, when the 

 case of Asclepias was alone known.* It is, however, 

 a wonderful sight to behold the tubes directing them- 

 selves in a straight line to the stigma, when this 

 is at some little distance from the anthers. As 

 soon as they reach the stigma or the open passage 

 leading into the ovarium, no doubt they penetrate it, 

 guided by the same means, whatever these may be, 

 as in the case of ordinary flowers. I thought that 

 they might be guided by the avoidance of light : some 

 pollen-grains of a willow were therefore immersed in 

 an extremely weak solution of honey, and the vessel 

 was placed so that the light entered only in one di- 

 rection, laterally or from faelow or from above, but the 

 long tubes were in each case protruded in every 

 possible direction. 



As cleistogamic flowers are completely closed they 

 are necessarily self-fertilised, not to mention the 

 absence of any attraction to insects; and they thus 

 differ widely from the great majority of ordinary 

 flowers. Delpino believes f that cleistogamic flowers 

 have been developed in order to ensure the production 

 of seeds under climatic or other conditions which tend 



* The case of Asclepias was de- pollen-masses whilst still within 



scribed by R. Brown. Baillon as- the anthers, in three widely distinct 



serts (' Adansonia,' torn. ii. 1862, Orchidean genera namely Aceras, 



p. 58) that with many plants the Malaxis, and Neottia : see ' The 



tubes are emitted from pollen- Various Contrivances hy which 



grains which have not come into Orchids are Fertilised,' 2nd edit 



contact with the stigma ; and that p. 258. 



they may be seen advancing hori- f ' Sull' Opera la Distribuzione 



zontally through the air towards Jei Sessi nello Piante,' 1807, 



the stigma, I have observed the p. 30. 

 emission of the tubes from the 



