354 Transactions. — Botany. 



points of the lateral sepals (see I., Fig. B.), yet the slightest touch is sufficient 

 to cause it to move quickly up to the column, when it occupies the position 

 shown in Fig. C. On this movement of the lip the fertilization of the plant 

 depends. 



If we take a flower, and gently touch the lip, so as to cause it to perform 

 the motion just described, and then examine the position of the parts, we see 

 that each side of the flower is narrowed inwards in a curved line parallel to 

 the position now occupied by the margins of the lip, so that the posterior part 

 of the flower forms a chamber, to which the lip, resting against the wing-like 

 appendages of the column, is a tolerably close fitting door. 



Now let us suppose that an insect were to enter a freshly opened flower. 

 The only entrance is between the tips of the lateral sepals, and here the apex 

 of the lip is placed exactly where our visitor would probably alight. At first 

 the weight of the insect would most likely counteract the natural tendency of 

 the lip to move inwards, but as the insect crawls further into the flower, this 

 would have less efiect, until at length the irritability of the lip would enable 

 it to overcome the resistance ofiered, and to spring back to the column. If 

 no capture is made the lip soon regains its former position, but if the insect is 

 imprisoned it remains firmly appressed to the column while its prey continues 

 to move about. For the prisoner there is now only one mode of escape. This 

 is by crawling up the column, passing over the stigma and viscid rostellum, 

 and finally emerging from between the appendages of the column, directly in 

 front of the anther. This passage, however, is so narrow and confined that it 

 would not be possible for an insect to pass through without brushing against 

 the rostellum, and detaching portions of its viscid surface. If the insect were 

 now to touch the anther, and it is difficult to see how it can escape without 

 doing so, one or more of the pollen-masses, lying loose in their cells, would 

 become glued to the viscid matter on the insect's back, and consequently be 

 withdrawn from the flower. To understand the mode of fertilization we have 

 now only to suppose that the insect, with the pollinia attached to it, visits 

 another flower, and is again imprisoned, when it is evident that in its eflbi-ts 

 to escape it would pass over and in front of the stigma, which is sufficiently 

 adhesive, when touched, to draw ofi" a portion of a pollen-mass, or even a whole 

 one, from the back of the insect. 



After careful and repeated examinations of living plants, I adopted this 

 view of the fertilization of P. trullifolia as the only one explaining the various 

 facts I had collected ; but, in order to satisfy myself that the lip really plays 

 the important part I had supposed, I selected twelve flowers which were just 

 expanding, and removed that organ from the whole of them. After a week or 

 two, when they had closed and commenced to wither, I gathered them and 

 examined their stigmas and pollinia. Not one flower was fertilized, and not 

 a single pollen-mass had been removed. 



