Dixon — On the Vegetative Organs of Vanda teres. 453 



stage between those of V. teres and D. teretifoUum. This orchid is 

 Brassavola Hadweni. The leaves of this orchid resemble those of the 

 other two in being terete. They are cylindrical, and taper to a fine 

 point. The upper surface of this leaf is a very deep, narrow groove 

 running down the leaf, so that in transverse section we see what looks 

 like a deep cut opposite the large midrib (PL xiv,, fig. 32). Near 

 the apex of the leaf this groove grows shallower (fig. 31), and finally 

 at the extreme point of the leaf is absent, so that the outline of the 

 transverse section at this part is quite circular (fig. 30). So far, this 

 leaf represents the stage in which the upper surface has become a deep, 

 narrow groove, the sides of which come into contact towards the base. 

 But in the same leaf we have only to examine sections taken lower 

 down to find the fusion of the edges of the groove to form an enclosed 

 canal (fig. 33). In such a transverse section near the base of the 

 leaf, where it is sheathed by a membraneous scale leaf, we find that 

 there is no longer a deep infolding of the upper surface, but instead a 

 narrow slit deep in the tissue of the leaf. This slit, as we trace it 

 downwards, becomes more and more nearly central in transverse 

 sections. 



From the examination of these orchids we can form an interesting 

 series of the possible stages passed through, to arrive at such a 

 peculiar structure as that found in the leaf D. teretifoUum. 



The leaf of V. suavis is a flat leaf, the rigidity of which is greatly 

 increased by the strong V-shaped fold. It has no well-defined petiole, 

 and its attachment at the node extends almost completely round the 

 stem. Supposing now that the opposite edges of the petiole met round 

 the stem and fused together, the tubular petiole of V. teres would be 

 attained, and by the Y-shaped fold being pressed together and fusing 

 across, and by the leaf becoming simultaneously fleshy, the form of 

 the lamina of V. teres would be arrived at ; the small groove repre- 

 senting the folded upper surface of V. suavis. The next stage 

 would be represented by a leaf such as that of B. Hadweni^ in 

 which the deep channel-like upper surface dips down, towards the 

 base of the leaf, into the tissue of the leaf and forms a passage con- 

 tinuous with the tubular petiole, shutting in the apex of the stem 

 from the exterior except for this narrow tube. By a continuation 

 of modification in this direction a leaf like that of D. teretifoUum 

 might be formed in which a capillary passage passes axially down the 

 leaf to a space included by the petiole over the apex. The history of 

 the development of this last, the extreme case, is as it were reca- 

 pitulated by the leaf of B. Hadweni, for the apex of a leaf being 



