CUAP. IX. GRADATION OF ORGANS. 251 



more viscid matter. In Habenaria clilorantha the 

 exterior surface is liigbly viscid, but still closely re- 

 sembles, under the microscope, the exterior membrane 

 of Epipactis. Lastly, in some species of Oncidium, 

 iSic, the exterior surface, which is viscid, differs, as far 

 as appearance under the microscope goes, from the 

 underlying viscid layer only in colour; but it must 

 have some essential difference, for I find that, until 

 this very thin exterior layer is disturbed, the under- 

 lying matter remains viscid; but, after it has been 

 disturbed, the underlying matter rapidly sets hard. 

 The gradation in the state of the surface of the ros- 

 tellum is not surprising, for in all cases the surface is 

 cellular in the bud; so that an early condition has 

 only to be retained more or less perfectly. 



The nature of the viscid matter differs remarkably 

 in different Orchids: in Listera it sets hard almost 

 instantly, more quickly than plaster of Paris ; in 

 Malaxis and Angrsecum it remains fluid for several 

 days ; but these two states pass into each other by 

 many gradations. In an Oncidium I have observed 

 the viscid matter to dry in a minute and a half; in 

 some species of Orchis in two or three minutes; in 

 Epipactis in ten minutes; in Gymnadenia in two 

 hours ; and in Habenaria in over twenty-four hours. 

 After the viscid matter of Listera has set hard, neither 

 water nor weak spirits of w ine has any effect on it ; 

 whereas that of Habenaria hifolia, after having been 

 dried for several months, when moistened became as 

 adhesive as ever it was. The viscid matter in some 

 species of Orchis, when remoistencd, presented an 

 intermediate condition. 



One of the most important differences in the state 

 of the rostellum ifi, whether or not the pollinia are 

 permanently attached to it. I do not allude to thjse 



