454 Proceedings of the Royal Irish Academy. 



formed first, in a sense represents an earlier stage than the middle, 

 and the middle than the base ; and we have seen that in the case 

 of this plant a leaf which is at the apex circular in section when 

 traced downwards becomes grooved, the groove lower down becomes 

 a narrow channel, and finally the channel becomes a closed passage, 

 which as we pass down into the youngest {i.e. last formed in the 

 ontogeny of the plant) part of the leaf takes up a more and more 

 axial position in the leaf. 



Root. 



The aerial roots of Vanda teres exhibit the characteristic structure 

 of the aerial roots of other orchids. They are completely covered by a 

 continuous sheath of tracheides, which have the usual fibrous thicken- 

 ings on their walls. In transverse sections (fig. 11) we can see that this 

 velamen is composed of two layers ; the outer, formed of these typical 

 tracheides, is uniformly one cell thick ; the inner is formed of much 

 more strongly thickened tracheides. The walls of this layer have not 

 the fibrous markings of the outer layer, but have pits with rounded out- 

 lines in their thick wall. In the inner layer, seen in transverse section, 

 there are frequent tangential walls, so that it is impossible to say 

 whether it is one or two tracheides deep. Bordering immediately on 

 the velamen is an esodermis composed of a single layer of cells, very 

 strongly thickened on their outer tangential and radial walls, while 

 their inner tangential wall is not thickened. The inner surface of the 

 outer thickened tangential wall is cuticularized. At intervals in the 

 exodermis, are passage cells, the walls of which are not thickened, and 

 which contain protoplasm and a large nucleus. Immediately outside, 

 and covering over the passage cells, is a dome formed of thin-walled 

 cells, which project out into the velamen. These cells also contain pro- 

 toplasm. ( Cf. Leitgeb, Denkschr. d. Akad. d. "Wiss. zu Wien, math- 

 naturw. CI. Bd. xxiv. p. 179, 1864). Within the exodermis is the 

 cortex, the cells of which have smooth thin walls and contain chlorophyll. 

 Here and there in this region are to be found cells with peculiar fibrous 

 thickenings (fig. 10). These thickenings are found almost exclusively 

 on that part of the cell-wall which borders on an intercellular space, 

 and would probably have the effect of checking rapid evaporation fi'om 

 these cells into the intercellular spaces, a contrivance which would, no 

 doubt, be of use to this plant. The cortex is bounded on the inside by 

 an endodermis, composed of cells with strongly thickened walls, opposite 



