MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 



299 



impenetrable by water, are two very small orchids, of which one, Bolbophyllum 

 m^nutiss^mum ) grows in company with mosses on blocks of sandstone and on the 

 bark of trees in the rocky ravines near Port Jackson, and on the Richmond River 

 on the east coast of Australia; the other, Bolbophyllum Odoardi, lives in similar 



Fig. 69. Stomata in the Furrows of Green Stems. 



1 Branch of Cytisus radiatus ; natural size. 2 Portion of a branch ; xlO. Cross section of this branch; x 30. Part of the 

 same section; x!50. 5 Branch of Casuarina quadrivalvis; natural size. Portion of a branch; x8. * Cross section of 

 this branch; xSO. Part of the cross section ; X130. 



situations in Borneo. Both have a filamentous rhizome from which spring rootlets 

 (from 2 to 5 mm. long and 0'3 mm. thick), arranged in pairs, by which they attach 

 themselves to the stone and the bark of trees. Above the origin of each pair 

 of rootlets is a little disc-shaped tuber, from 1J to 3 mm. in diameter, and J mm. 

 thick, with an aperture on the upper surface, scarcely ^ mm. broad, leading into a 

 hollow chamber within the disc -shaped tubers, about 0*5 mm. broad and 0*1 mm. 

 high (see figure 70). The leaves of Bolbophyllum minutissimum are reduced 



