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THE FERTILTZATIOX OF CEPHALANTHEIU. 



511 



The rertilizafion of CepJialanthera, Jl\c\ 



By (Jolonel M. J. GodferYj F.L.S. 



1 



[Read IGtli Juno, 1921.] 



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Cefiialantiiera is a genus of tlie Orcliidacea?, of which three species grow 

 in Britain — gmndljlora^ ensifoUa^ and rubra. It will help us to undershmd its 

 fertilization if we first glance at that of JLjApactis^ the nearest allied genus^ 

 as seen in _£'. latifolia^ a common British species, fertilized by wasps. The 

 pollinia are Luilt up of an innnenso number of tetrads, each consisting of four 

 pollen-grains, closely compressed. Projecting forwards over the edge of 

 the stigma is the rostellum, or viscid gland, a milky-looking ball of very 

 adiiesive matter, to the back of which the two pollinia aie already firmly 

 attached when the flower opens. 



This is so placed in the flower that u wasp coming to suck nectar is sui'o 

 to touch it. Immediately the rostcllunij with the two pollinia attached, 

 adheres to the insect's head, and when it enters anotlu^r flower the pollinia 



come in contact with the stigma, and leave quantities of pollen on its viscid 

 surface. 



The tetrads soon split up into tlieir component pollen-grains, each of whicli 

 emits a pollen-tube, which grows downwards till it reaches the ovules and 

 fertilizes them.^ This maybe called feriilization by the principle of the viscid 

 gland^ a method which, in endless variety of detail, prevails almost through- 

 out tlie Orchidacepe, except in Diandric- The variations are often extremely 



curious, but the root-principle is the same in all — the provision of a niass of 

 very adhesive material by the rostellum, which cements the pollinia to a 

 visiting insect. 



When we try to find out how Cepltalanthera is fertilized, we are met at 

 the outset by an apparently insu])orable difficulty — there is no rostellum 

 whatever. The conspicuous flowers are well-suited to attract insects, lut 



' there is a complete absence of the usual mechanism for the removal of the 

 pollinia* 



Darwin investigated the fertilization of C\ grandijlora^ the commonest 

 British species. He found that the curved pollinia lean forAvard and rest on 

 the upper edge of tlio stignui, to wluLdi they become anchored by pollen-tubes 

 a little below their centres, their upjier parts projecting forwards over the 

 edge of the stigma. Jhese, whicli he likened to two pillars of pollen, 

 he believed became broken down by minute insect?, so that fragments fell 

 down and adhered to the sticky surface of the stigma, the flower being thus 



