VELAMEN. 



227 



lamina, and in the pitchers 2-3 layers of cells below the surface. They are never 

 continuous with the vascular bundles. In the stem they are all arranged parallel to 

 its axis ; in the leaf, at least in the wall of the pitcher-shaped portion, they point 

 irregularly in different directions. 



In the base of the leaf of the Isoeteae ^ are found series of short spiral tracheides, 

 having the same form as those of the xylem in the stem of these plants : they occur 

 in the dense mass of parenchyma at the point of insertion of the membranous ligula, 

 called by Braun the Glossopodium. They extend from the upper and lower margins 

 of this body almost horizontally to the inner surface of the base of the leaf: those of 

 the upper side towards the posterior wall, those of the under side towards the mem- 

 branous lip-like lower margin of the depression in which the ligule is seated. They 

 have no connection with the vascular bundle of the leaf. 



Sect. 56. A continuous layer of air-containing tracheides covers, as a sheaik or 

 ■uelamen, the aerial roots of epiphytic orchids, which 



in this respect resemble those of some other plants, A 



especially Aroideae. 



The tracheal sheath of the roots of Orchids 

 is produced from the layer of dermatogen, which, 

 according to Treub '■', in Vanilla and Stanhopea, is 

 differentiated close behind the growing-point from a 

 common initial group for the root-cap and the body 

 of the root : this I found to be the case in Vanda 

 furva, while in species of Oncidium (Fig. 90, 91) 

 the dermatogen passes over the growing-point as 

 a distinct layer between periblem and calyptrogen. 

 The simple layer of the periblem, adjoining the 

 dermatogen internally, develops into the endoder- 

 mis, which consists of longitudinal rows of alter- 

 nately elongated and short cells (comp. p. 125). 



The very delicate cuticle which is present at 

 first, i. e. where the root emerges from the root- 

 cap, is absent over the mature outer surface, or 

 at least cannot be proved to be present as a con- 

 tinuous skin. 



The sheath of tracheides remains in some 

 few cases a single layer (Vanilla planifolia, aphylla, 

 Sarcopodium Lobbii, Cirrhopetalum Wallichii) : in 

 most cases it is cut up by corresponding divisions, 

 which begin behind the growing-point, into several 



layers, numbering according to the species 2, 3, 6, or 18 (Cyrtopodium spec). 

 All its elements are in uninterrupted connection with one another (comp. Figs. 90, 

 91, <f). The single tracheides are approximately iso-diametric, or slightly elongated 



Fig. 90. — species of Oncidium ; aerial root, 

 sliglitly magnified. A axile longitudinal section 

 through the apex, c — c root-cap, p depressed 

 centre of the growing-point, d sheath of tra- 

 cheides, h endodermis, /" vascular bundle, r cor- 

 tical parenchyma. B transverse section through 

 a mature part ; lettering as in A. 



' Mettenius, Linnoja, 1847, p. 272. — Hofmeister, Beitr. p. 151 

 dinien, Berliner Acad. Monatsbr. 1863, p. 571. 

 ' I.e., compare p. 10. 



9 2 



-A. Braun, Isoeten d. Ins. Sar- 



