VELA MEN. 



229 



of the same root, and in the several walls of one and the same tracheide. In most 

 cases the walls are thickened by spiral fibres, which in some plants run exactly 

 parallel (Sarcanthus rostratus, Gongora Jaenischii, Brassia maculata, Cattleya 

 Mossias), or leave slits between them (Oncidium pulvinatum, flexuosum, sanguineum), 

 or form large meshes (Epidendron elongatum, Brassia caudata) : or in other cases 

 are arranged in band-like groups (Cyrtochilum bictoniense). According as these 

 fibres are in close juxtaposition (Oncidium flexuosum, sanguineum, Cymbidium 

 ensifolium), or are further apart from one another (Maxillaria tricolor, Camaridium 

 ochroleucum), the slits and meshes are smaller or larger. Further, since in many 

 cases the fibres of two contiguous walls cross, the superposed slits and meshes are 

 also crossed. Not uncommonly the spiral fibres, which usually traverse the wall 

 obliquely, but often appear (in transverse sections) radially arranged, run quite 

 irregularly, in which case they are however sparsely distributed and branch re- 

 peatedly, and then either continue their course independently, or again unite later to 

 form broad bands (Renanthera matutina, Phalgenopsis grandiflora, Saccolabium 

 Blumei). In other cases the spiral-fibrous thickening disappears entirely, and there 

 are only solitary slits to be seen, which are still arranged in spiral lines (Angraecum 

 subulatum, outermost layer) ; but not uncommonly this spiral arrangement also 

 disappears, and a purely reticulate thickening is found (Dendrocolla teres, Sobralia 

 decora, Vanda furva). In some though less frequent cases the walls are again quite 

 regularly thickened, and only present more or less numerous pits (Angrjecum 

 subulatum, second layer) ; often the thickening layers are only developed at the 

 corners (Sarcopodium Lobbii, Cirrhopetalum Wallichii), or the walls are without any 

 thickening at all, and are quite thin (Trichotosia ferox ; Angraecum subulatum, third 

 layer, Leitgeb). 



These examples may serve to show the variety of the details, while reference 

 may be made to the further special descriptions and figures of Oudemans and 

 Leitgeb. What has been said of the triple sheath of Angraecum subulatum shows at 

 the same time the variety of form of thickening, which often occurs in successive 

 layers. Though the conditions in this respect also are very variable, the rule still 

 holds that both in forms with one and with many layers, at least the outer and the 

 inner surfaces are characterised by special thickening of the membranes. 



At those points where the spiral or netted fibres separate widely in a slit-like 

 manner from one another, the wall-surfaces which have no fibres are not uncommonly 

 perforated ^, whether they be on the free surface or in the body of the sheath. Thus 

 in the latter case, speaking accurately, the tracheides are united to form vessels. 

 Where the sheath consists of several layers, the elements of the outermost layer often 

 grow out as papillae or sac-like hairs, a phenomenon which also appears in the one- 

 layered sheath. In those cases known, all the elements of the surface are not engaged 

 in the formation of hairs. The latter sometimes occur on roots which protrude 

 freely into the air, as Leitgeb found in seventeen species of different genera (in Eria 

 stellata the felt of hairs, which is usually dense, is quite absent if the roots grow in 

 moss or earth) : while in other cases the formation of hairs appears only when the 



' Von Mohl, Verm. Sclirifleii, p. 322 (Epidendron elongatum). 



