226 



Darwin noted that "flowers icJien they first arrived were not 

 sensitive, btit after the spiJces had stood for a daij or tico in loater they 

 became sensitive. Whether this is oiving to fuller maturity or to the 

 absorption of water, I Jcnoiv not." 



There is reason to think that both maturity and absorption have 

 something to do with the so-called irritability, for if the pollinia are 

 fully ripe the disturbance of the anther-cap is quite sufficient to 

 cause the expulsion of the pollinia (as we have proved by experiment 

 on numerous occasions) without in any way touching the " antennae " 

 or " horns." 



On the other hand, the anther-cap can be wholly removed, and 

 the pollen masses will not be expelled, but this can only be effected 

 when the parts are immature. In Darwin's description of C. saccatvm 

 he states that the right hand antennee hangs downwards paralysed 

 and functionless. This he repeats twice later on in the same work ; 

 while at p. 225 he states that a touch of the right hand antenna; 

 causes the pollinia to be instantly ejected. This statement is hard 

 to unravel, but it clearly does not lend itself in direct evidence of 

 special irritability. 



A series of sections of flowers taken at different stages of develop- 

 ment, appear to show that the expulsion of the pollen does not 

 depend upon any special irritability, but upon mechanical action alone. 



The pollen masses, the caudicle and the gland or " viscid disk" 

 are seen to be held in place by tissue which, as it matures, granulates 

 and forms a " layer of separation," or " absciss layer " which allows 

 the division to occur ; but until this tissue has properly matured no 

 rupture can or does take place. The antennae are seen to be merely 

 a prolongation of the material forming the edges of the stigmatic pit, 

 curled into cylindrical form. A part of this curl grips or holds the 

 margin of the caudicle, at the base of the antennse, and the antennae 

 at the period of anthesis become turgid, stiff and non-elastic. In 

 this state they furnish levers which are amply sufficient to cause a 

 disturbance of the grip they hold upon the margin of the caudicle, 

 and to produce a vibration or disruption of the separation layer, on 

 being slightly touched. 



The horns or points of the antennae, do not stand near to. but 

 far back from the point of dehiscence of the anther, and the point 

 of the coliTmn or rostellum is the first place upon which an insect 

 would alight when visiting these flowers, if standing in certain posi- 

 tions, but it would first have to travel over the easily displaced 



