Chap. IX. GRADATION OF ORGANS. 247 



CHAPTER IX. 



GRADATION OF ORGANS, &C. — CONCLUDING REMARKS. 



Gradation of organs, of the rostellum, of the pollen-massefi — Formation 

 of tlie caudicle — Genoalogical affinities — Secretion of nectar — 

 Mechanism of the movement of the pollinia — Uses of the petals — 

 Production of seed — Importance of trifling details of structure — 

 Cause of the great diversity of structure in the flowers of Orchida 

 — Cause of the perfection of the contrivances — Summary on insect- 

 agency — Nature abhors perpetual self-fertilisation. 



This chapter will be devoted to the consideration of 

 several miscellaneous subjects which could not well 

 have been introduced elsewhere. 



On the gradation of certain Organs. — The rostellum, 

 the pollinia, the labellum, and, in a lesser degree, 

 the column, are the most remarkable points in the 

 structure of Orchids. The formation of the column 

 and labellum, by the confluence and partial abortion of 

 several organs, has been discussed in the last chapter. 

 With respect to the rostellum, no such organ exists 

 in any other group of plants. If the homologies of 

 Orchids had not been pretty well made out, those 

 who believe in the separate creation of each organism 

 miffht have advanced this as an excellent instance of 

 a perfectly new organ having been specially created, 

 and which could not have been developed by suc- 

 cessive slow modifications of any pre-existing part- 

 But, as Eobert Brown long ago remarked, it is not a 

 new organ. It is impossible to look at the two groups 

 of spiral vessels (fig. ?0) running from tlie bases of 



