34 C.ENKHAL REVIEW OV THE ORCHIDE.%;. 



US from reproducing^ we are coastrained to give all the salient facts 

 in an abbreviated form witlioutj we trust, obscuring their import. 

 In doing so we wish to express our obligations to Mr. Francis 

 Darwiu and to the publisher Mr. John Murray, for the kind manner 

 in which they have allowed us to use the diagram from the 

 Fertilisation of Orchids, and also other illustrations from the same 

 work which are duly noted in their respective places. 



Xo grcui]! of (irgauir beings, Mr. Darwin observes, can be well 

 understood till their homologies have been made out, and in no case 

 is this more applicable than to orchids. The importance of the science 

 of homology rests on its giving us the key-note of the possible amount 

 of difference in plan within any group ; it allows us to class under 

 proper heads the most diversitied organs ; it shows us gradations which 

 would otherwise have lieen overlooked ; it explains many monstrosities ; 

 it leads to the detection of obscure and hidden parts or mere vestiges, 

 and shows us the meaning of rudiments. Thus guided, the naturalist 

 sees that all homologous points or organs, however much diversified, 

 are modifications of one and the same ancestral organ ; in tracing- 

 existing gradations a clue is gained as far as that is possible, in tracing 

 the [trobable course of modifications during a long line of generations. 



The mode of investigation pursued in order to make out these 

 homologies is either by tracing their embryological development when 

 that is possible ; or by the discovery of organs in a rudimentary 

 condition ; or by tracing through a long series of beings, a close 

 gradation from one part to another until the two parts or organs 

 employed for widely ditferent functions and most unlike each other 

 can be joined h\ a succession of short links. Applying these methods 

 of investigation to orchid fiowers and guided by the general structure 

 of monocotyledonous plants, Dr. Robert Brown first propounded the 

 hypothesis that an orchid flower properly consists of three sepals, 

 three petals, six anthers in two circles or whorls of three each, and 

 of three pistils, one of which is modified into the rostellum. These 

 fifteen organs are arranged as usual alternately three within three in 

 five whorls. 



The relative position ol all these (jrgaus is shown in the following 

 diagram, Avith the excei)tion of A2, A3, which should stand outside 

 the crescent representing the labellum, a little nearer the triangle. 



The little circles show the position of the spiral vessels (fibro- 

 vascular bundles) ; s s, stigmas ; sr, stigma modified into the rostellum, 

 Al, fertile anther of the outer whorl; A2, A3, anthers of the same 

 Avhorl ; al, a2, rudimentary anthers of the inner whorl (fertile in 

 Cypripedium) genei'ally forming the clinandrium ; a'3, third anther of 

 the same whorl, when present, forming the front of the column. 



