248 GRADATION OF ORGANS. Chap. IX. 



the midribs of the two lower sepals to the two lower 

 stigmas, which are sometimes quite distinct, and then 

 to look at the third group of vessels running from the 

 base of the mid-rib of the upper sepal to the rostellum, 

 which occupies the exact position of a third stigma, 

 and doubt its homological nature. There is every 

 reason to believe that the whole of this upper stigma, 

 and not merely a part, has been converted into the 

 rostellum ; for there are plenty of cases of two stigmas, 

 but not one of three stigmatic surfaces being present 

 in those Orchids which have a rostellum. On the 

 other hand, in Cypripedium and Apostasia (the latter 

 ranked by Brown in the Orchidean order), which are 

 destitute of a rostellum, the stigmatic surface is trifid. 

 As we know only those plants which are now living, 

 it is impossible to follow all the gradations by which 

 the upper stigma has been converted into the rostellum ; 

 but let us see what are the indications of such a change 

 having been effected. With respect to function the 

 change has not been so great as it at first appears. 

 The function of the rostellum is to secrete viscid matter, 

 and it has lost the capacity of being penetrated by 

 the pollen-tubes. The stigmas of Orchids, as well as 

 of most other plants, secrete viscid matter, the use of 

 which is to retain the pollen when brought to them by 

 any means, and to excite the growth of the pollen- 

 tubes. Now if we look to one of the simplest rostel- 

 lums, — for instance, to that of Cattleya or Epidendrum, 

 — we find a thick layer of viscid matter, not distinctly 

 separated from the viscid surface of the two confluent 

 stigmas : its use is simply to affix the pollen-masses to 

 a retreating insect, which are thus dragged out of the 

 anther and transported to another flower, where they 

 are retained by the almost equally viscid stigmatic 

 surface. So that the oflice of the rostellum is still to 



